Thus far in this survey plant life has been the center of interest. This is partly a convenience in dealing with complex materials and partly because there is so large a body of pertinent literature. For animals both large and small, there is relatively little literature in a form suitable for this survey. In 1937 the American edition of Richard Hesse, Ecological Animal Geography was offered in English. It was translated from the German and in part rewritten by W. C. Allee and Karl P. Schmidt, with a preface stating that its original publication in 1924 marked a new era as it was the first attempt to apply ecological principles to the study of animal distribution on a world-wide scale. in reviewing a group of biology textbooks in 1942, Dexter pointed out again that in ecology, zoology lagged in both research and teaching. Hesse devoted one chapter to the characteristics of animals of the "dry, open lands" of the world. Animals of such areas were described as tolerant of dry air, relatively independent of water, or regular watering, and provided with means of protection against or resistance to wind and to sharp fluctuations of temperature, illustrated by the burrowing habits of rodents, snakes, and ants. To both birds and animals he attributed conspicuously the cursorial habit, quiet rather than noisy behavior, dependence upon sight rather than sound for protection, the flocking habit, or life in communities. The grass and grain eaters were the key industry animals of the native grasslands, converting vegetation into food for the carnivorous animals. Insect species were numerous and their numbers enormous, with three types dominating; the grasshoppers, ants, and termites. In the economy of nature, the grasshoppers were stressed as eaters of the dry, hard vegetation, turning it into live form. Rodents were said to exceed all other mammals in both species and numbers, consuming 75 per cent of all the food available.
One of the earliest attempts at a scientific description of the American prairie fauna was that of Allen (1871). His (1870, 1871) was the typical reaction of the forest man to the grassland:
With all the beauty and the novelty of the primal flora of the prairies, the traveler, after a few weeks of constant wandering amid their wilds, is apt soon to experience a monotony that becomes wearisome, the full degree of which he scarcely realizes till the green sward and the varied vegetation of cultivated districts again meet his eye.
He held to the theory, reiterated by Chapman (1931), that "the diversity of the animal and vegetable life of a given region ... [is] dependent upon the diversity of its physical features," using the plains, the prairies and the forests to illustrate the three degrees of variation in topography, flora and fauna. Upon entering the prairie from the eastward Allen said that the mammals of the forest did not disappear but were restricted to wooded areas and therefore were relatively less prominent. The prairie mammals met there were markedly different. In transition the animals of the two regions mingled. The forest lynx, panther, and bear became rare, while rodent, skunk, mink, fox, and wolf were relatively abundant and together with the prairie species the kit-fox, badger, prairie wolf, and striped skunk, the ratio of these types of animals to the whole population was conspicuous. Among the rodents met for the first time were the pouched gopher and two species of ground squirrels of the prairie. The same principle applied to the birds, new ones named were the Cerulean Warbler, Swallow-tailed Kite, Prairie Hen, Turkey Buzzard, and Sand-Hill Crane. Reptiles and mollusks were said to have been rare, on ac-count of annual prairie fires. Data on risk were not available. In respect to insects their characteristics were referred to as similar to the flora upon which they depended so largely; "No country, however, it is hoped, is richer in Orthoptera (grasshoppers), either in species or individuals." He concluded the discussion with the verdict that in prairie fauna, as in flora, "there is a simplicity and uniformity that gives to both a comparatively low and uniform character" in contrast with forest life. Among the first of the twentieth century ecologists to study the fauna of the prairie, Ruthven (1908) insisted that "The prairie is the most interesting biotic region of North America," but emphasized that "the vertebrate life has never, as a whole, received examination." His expedition into Iowa in 1907 led to the conclusion drawn from comparative lists of plains, prairie and forest birds, gophers, and snakes, that
Most of the forms which inhabit the prairie region either extend also into the eastern forest region or into the plains region or rarely both, few being confined to the prairie region.... The intermediate character of the new environmental conditions makes the prairie region an extensive area of transition between the plains and eastern forest regions, but that the environmental conditions are not either intensive or extreme enough to mold the forms into a peculiar fauna.
Shackleford (1929) made studies of the small animals of the high and low prairies of Illinois. Those of the former migrated to some extent to the latter during dry seasons, but those of the low prairie did not migrate to the high prairie. The animal population exhibited more uniformity on the high prairie, while on the low, it varied from place to place from year to year. Similar determinations have not been made systematically for other portions of the grassland, but topographical, soil, and vegetational variations were probably accompanied by analogous but widely varied fluctuations in animal populations. A study of the eastern house wren, by Kendeigh (1934), indicated a wintering range extending as far as eastern Texas and a breeding range extending northward and westward to Indiana and eastern Wisconsin. The range of the western house wren lay west of that line, belonging primarily to the grassland, wintering from Texas and California south into Mexico, and breeding as far north as southern Canada. In his study of rodents in California, Grinnell (1923) found in that state seven genera with burrowing habits, or about one-fourth of all California mammal species. In tracing the extent of distribution eastward he indicated the first break at a line or zone running north and south at approximately the 100 meridian. This marked the principal area occupied by the same forms. From the 100 meridian eastward the representation in genera and species diminished until only two species of one genus (Citellus, ground squirrels) were recorded in Indiana, which, according to vegetation distribution, lies at the eastern tip of the prairie peninsula. The area between is the tall grass and the mixed grass prairie. The forest regions had their own peculiar population. Grinnell attributed these limitations on burrowing rodent distribution to atmospheric humidity, rainfall, sharp alternations of dry and wet seasons and probably "a relatively greater abundance of plants with nutritious roots or thickened underground stems (corms, root stocks)."
The prairie dog possessed a particular significance to the grassland as a regional indicator. The Rocky Mountains marked its western boundary of distribution, except in the southwest, where it extended as far west as Arizona. Its eastern limit was near the 97 meridian in Kansas and Nebraska bending somewhat west of that line in Texas, but in the North following approximately the Missouri river through the Dakotas and Montana (Merriam, 1901) These animals had multiplied and spread as a result of increased root supply from agricultural crops, and from the killing of their enemies by man, the coyotes, kit foxes, badgers, ferrets, weasels, minks, owls, and snakes (Merriam, 1901; Shelford, 1940). The coyotes, represented by twelve species, had a wide range of distribution from the central Mississippi valley to the Pacific coast and from Costa Rica to Athabasca (Lantz, 1905). Jack rabbits were present with two species having as an eastern limit of distribution the western part of the tall-grass prairie in Minnesota, Iowa, and eastern Kansas and extending westward to the Pacific coast in California and to the forest regions of the Pacific Northwest. The white-tailed species occupied the northern part extending into the Canadian plains, and the black-tailed species occupied the southern part. In the middle was an area of over-lapping in southern Nebraska and in northern Kansas, extending westward to Nevada (Palmer, 1897). Different species of the Tetraonidae (ptarmigan, prairie chicken and grouse) occupied tundra, forest, and grassland separately or in combination in varying regional distribution (Pitelka, 1941). The greater prairie chicken (Tympanuchus cupido) occupied the area from the forest edge of the prairie peninsula westward into but not through the high Plains, with a southern boundary of abundance at about the south line of Kansas, and scattering occurrence farther south. The lesser prairie chicken occupied southern Kansas, northwestern Oklahoma, the Pan- handle of Texas, and the eastern edge of New Mexico. The sage grouse (Centrocercus urophasianus) ranged eastward into the short-grass plains and westward into the desert of the Great Basin and northwest into the Pacific prairie. In much reduced numbers they still populate much of the designated area of the grassland. Some other related species were not restricted to a single form of vegetation, particularly the sharp-tailed grouse, which ranged as far south as 37 in the grassland and desert and occupied northern forest as far as the tundra regions.
For subdivisions of the grassland, the animal distribution can be analyzed in more detail. For the northern grassland Visher (1916) made a study in what he called biogeography in which he described all major forms of life together; vegetation, large animals, birds, smaller animals, and insects. To emphasize the variety, rather than the uniformity of the region, he broke it down into its vegetational and soil types, showing the wide range of difference in each, animal life as well as in grasses and forbes: The buffalo-grama grass association; the needle grass, or sandy loam steppe, association; the wheat grass, or clay steppe, association; the bunch-grass, or dry soil steppe, association, of which there were two phases, the rugged areas, slopes, not sandy, river bluffs, and moraines; and secondly, the sand dune areas; and the low shrub group association. This kind of treatment is particularly important to counteract the prevailing misconceptions of the uniformity and monotony of the grassland. The only thing that was uniform about the grassland was that the vegetational covering was grass as distinguished from trees, but within limits of the grasses as plant life, there was a great variety of species and combinations of grasses and forbs. The grassland had fewer layers of vegetation in its structure than the forest and possibly it was this characteristic that gave the impression to the forest man of monotonous uniformity. In describing the animals of the northern grassland, Visher said that the mammals possessed two or more of a list of characteristics; ability to run swiftly, the burrowing habit, acute long-range vision, coloration adaptation of gray or tawny, ability to do without much water, daily activity confined to early morning, evening, or sometimes nighttime, ability to hibernate, lack of gregariousness except for protection or warmth in some cases as antelope and bison, and largely herbivorous habits. A similar method was used in describing birds which possessed two or more characteristics; ground nesters, singing on the wing, louder calls than forest species, social flocking less prominent, ability to withstand a strong wind, females and nestlings mostly protectively colored, ability to withstand intense heat and scarcity of drinking water, and acute long-range vision. In North Dakota, as in most of the low-rainfall grassland area, the burrowing forms of animals are conspicuous and in this instance gave to the state the nickname "Flickertail" state. The Franklin ground squirrel occupied only the northeastern-most portion of the state with a western boundary to its range near a line from Mouse river running southeast down the Dakota river (Bailey, 1926). The Mississippi pocket gopher area lay east of a line from Devil's Lake down the Dakota river while the Dakota pocket gopher occupied the country west of that line. The Richardson ground squirrel, "the Flickertail", was limited to the country east and north of the Missouri river. The major race of the thirteen-lined ground squirrel was similarly limited but a pale western form occupied the area south and west of the Missouri river. The same river marked the eastern boundary of the plains form of the prairie dog in North Dakota. In respect to these species of animals the state was divided by the Dakota and the Missouri rivers into three ecological areas. The Dakota river lay between the 98 and the 99 meridian, but the Missouri river did not approximate any meridian. At the southern end of the grassland, the 98 meridian marked a fairly close approximation to the western range of some eastern animals or the eastern range of others (Bailey, 1905, 1928). The Texas woodpecker and the Texas rattlesnake territory lay just west of the 98 meridian with a western boundary between the 100 and 101 meridians, and a northern range extending just north of the Red river. Such plains animals as the bison, the plains race of the white-tailed deer, the plains jack-rabbit, and the plains prairie dog reached their western limit just east of the Pecos river watershed in New Mexico or just west of it (Bailey, 1931).
In the central grassland, it has been customary to designate the 97, the meridian of Fort Riley, Kansas, or some line or zone between that and 99 as the approximate division line between eastern and western forms. Blair and Hubbell (1938) listed 73 species, or 91 species and races of mammals in Oklahoma. Or the 73 species, 22 (30 per cent) were eastern forms that reach western limits within the state, and 22 species were western forms that reach eastern limits within the state. Six species were northern forms that reach southern limits, and two species were southern forms that reach northern limits in the state. No survey of similar completeness exists for Kansas and Nebraska, but Hibbard's (1944) revised viewpoint abandoned the 98-99 boundary in Kansas and proposed a pattern in which the eastern forms extended up the valleys of the principal streams somewhat west of such a line, and the western forms extended down the upland divides between the streams to points east of such a line. This is the same pattern of distribution for animals which Schaffner (1926) had designated for central plains vegetation and tended to bring ideas on animal distribution into harmony with Schaffner's suggestion of the coincidence of boundary between the tall-grass and the mixed-grass prairie at the eastern extent of the range of the prairie dog and the harvester ant. Clements and Shelford (1938) took over the idea from Schaffner. It fitted also into the finding of Shackleford (1929) relative to the influence of topography on faunal distribution between high and low prairie of Illinois.
In 1927 Hayes remarked that the "insect fauna of the prairie has been given so little consideration, ecologically, that it is with some hesitation that [this]... discussion is attempted." When Osborne's Meadow and Pasture insects was published in 1939, the point was stressed both by the author and the reviewers that little had been done with grassland insects. Osborne emphasized as a retarding factor, the "relative obscurity and constancy of the damage," and the conviction that nothing could be done about it. In reviewing Osborn's book, King (1940) emphasized that it was the first genuine treatise in the field and that the grassland comprised about 40 per cent of the land surface of the world as a natural or climax community besides the area planted to grass as an agricultural crop. The understanding of insects was important not only to the grasses, but to the crops subject to injury by insects from the grasses. Allen's general surveys of flora and fauna of the prairie, 1870 and 1871, included a brief mention of insects, especially commenting upon the number of grasshoppers. Allee (1927) pointed out that in some land communities, the most abundant and important animals were insects, although in land communities generally, Shelford (1931) held that mammals and birds were most important.
The grassland and the shrub transition zones, according to Shelford (1915), were the original habitat of the principal insect pests of the forage and garden crops and of small fruits and orchards. Native to the original vegetation of these areas, with the clearing of the adjoining forest and undergrowth and the planting of cultivated crops and fruits, the insects changed hosts and many multiplied as a result. In addition to the native grassland insects the central area was the crossroads of insect migration from northwest, southwest, southeast, and importations (Webster, 1903; Van Dyke, 1919; Hayes, 1924, 1927; Smith, 1925; Blair and Hubbell, 1938). Of Oklahoma's 254 species and races, 90 (35.4 per cent) were eastern forms which reached western limits to their range within the state; (35 per cent) were western forms with eastern limits within the state; 32 (12.6 per cent) were plains species extending into the state from north and south, but having both their eastern and western range limits within the state; 62 (24.4 per cent) reached northern or southern limits within the state (Blair and Hubbell, 1938). Riley county, Kansas, was designated by Faye (1924, 1927) as the ant center of the United States and Smith (1925) made a similar claim in respect to the Neuroptera and Mecoptera, the forms from the south direction meeting and overlapping in that vicinity. Hayes (1927) suggested that this might be true of other faunal groups when adequate information was available. Such a generalization may be too sweeping as applied to Riley county, or to Kansas because the Oklahoma data show a substantial meeting of north-south range limits within that state. A more substantial accuracy would be achieved probably by designating the central grassland (Oklahoma-Kansas-Nebraska) as the area of transition and such a picture would contribute much to the understanding of the striking variety and abundance of insect life in that area.
Climatic factors affected the several kinds of insects differently; wet years were favorable to the Hessian fly, but unfavorable to the chinch bug (Shelford and Flint, 1943), and the reverse. The westward diminution of rainfall probably acted as a limitation on the distribution of the Hessian fly (Hayes, 1927). The idea that temperature controlled the development of insects was challenged by Shelford (1926). Especially in unusual seasons, when control was most needed, this hypothesis was most likely to fail. The relation of insects to weed control was illustrated by the white beard tongue, a weed with an increase potential of 4000 fold. Insects destroyed 55 percent of the seeds within two months of the flowering of the plant, constituting the most important single factor in holding that weed in check to a point of approximately a single plant annually replacing its predecessor (Brandhorst, 1943). Chapman (1931) cited what must be an unusual example of an insect, normally unimportant, attacking and destroying an invading plant, thereby contributing to the maintenance of the integrity and stability of the biotic equilibrium. The cacti of the plains region multiplied rapidly during all prolonged drought periods, and gained unusual publicity during the depression-drought hysteria of the 1930s under the ministrations of those who were ignorant of the biological principles in operation in the grassland. The situation did focus attention, however, upon making the controlling factors the subject of investigation and scientific record, an opportunity which was practical for the first time as this was the first prolonged drought since the biological science were adequately developed and staffed to make such studies (Cook, 1942; Turner and Costello, 1942).
The cacti afford an unusually clear case of the coordination of a number of multiple factors in effecting the unstable equilibrium in nature. In an investigation centering around Hays, Kansas, of the relation of insects to the cacti (Opuntia humifusa), Cook (1942) found three kinds; one a bug similar in appearance to the box-elder bug (Chelinidea vittiger Uhl) which had little apparent importance, several species of mealy bugs (Dactylopius sp-) which were of greater importance, and a moth (Militara dentato) which seemed to be a decisive factor. In its larval stage, this moth occupied the central stems of the cacti, eating the plant from the inside. Moist warm weather was favorable to the rapid build-up of the moth population, and dry hot weather was unfavorable to its growth. Rodents, and especially jack rabbits, ate the cactus roots and scattered the seeds. Thus grass, cactus, animals, and insects interacted upon each other under fluctuating weather conditions, especially successive years of similar weather where the effects became cumulative.
Cacti and grass were not so much competitors or enemies as complementary to each other. In prolonged moist periods grass provided the soil cover, with a minimum of cacti; during prolonged dry periods unfavorable to grass the cacti took over and provided the soil cover. This occurred irrespective of the extent of grazing. It was associated with the weather fluctuation and was not the result of overgrazing. Grazed and improved tracts showed the same trend (Cook, 1942). The multiplying clumps of cacti held the soil from extensive blowing, catching in the clumps the dust and grass seed. As soon as favorable grass-growing weather returned the seed sprouted, the cactus clumps serving as a protective nurse crop to the new growth of grass (Malin, 1942, p. 25 note). The grass repaid the cactus by harboring insect life which attacked and killed the cactus, leaving the grass in full possession during the moist period. With the return of dry years the grass was weakened or died out and insects died also, permitting the cacti to multiply, and animals, particularly jack rabbits, being limited in other food and water supply, ate more cacti and this scattered the seed. Thus alternately a series of dry years built up the cactus phase and a series of moist years built up the grass phase of vegetation, both coordinated with animal and insect reactions. This emphasized how foolish it was during the 1930s to become excited over the supposed danger of the cacti permanently dominating the plains. Grasshoppers received more publicity than any other insect of the grasslands, but only occasionally were they an outstanding menace. The chinch bugs probably did more consistent and serious damage (Malin, 1944). The dramatic aspects of the swarming phase of the so-called Rocky Mountain locust in 1874 so impressed the public mind, ignorant of the characteristics of the swarm of insect, that tradition alleged frequent and periodic swarming and devastation (Hafen and Rister, 1941, p.429). A survey (Ball, 1937) of Colorado and Arizona grasshoppers established a record of 130 species in each state. Some 40 species were beneficial in attacking the worst weeds of the range, some 70 were not important, 5 or 6 were of major importance in attacking cultivated crops, and 10-12 in damaging range grasses. Of 40 species studied by Isely (1938), 37 were non-migratory and were usually rated as of no economic importance, from the standpoint of crop damage, but of this second group several species were positively beneficial as weed eaters, and the grass eaters had adverse significance only when there was a shortage of grass. To the mind of the average city dweller, conditioned by grasshopper scourge stories, the idea is novel indeed that a grasshopper could be beneficial. The use of grasshopper machines and poison made spectacular headlines, but dealt with results not causes and such benefits as they brought came after the damage was done in part.The grasshopper was subject to many predators; fungi and parasites, land animals and birds, and to some degree biological controls were possible (Ball, 1937; Sweetman, 1936). In irrigated districts, insects were more generally under control (Ball, 1937). In non-irrigated areas, insect damage was more serious, where control measures proved ineffective because of waste, eroded, or inefficiently managed land. Also, the non-irrigated regions involved much greater land areas, wider differences in environment and more variety in species (Ball, 1937). Some species of grasshoppers died if not provided with grass for root, while others, Melanoplus differentialis, possessed a wide range of adaptability to other plants (Isely, 1938). Climate, particularly rainfall, also seemed to be a determining factor in the distribution of some species according to studies in northeastern Texas (Isely, 1937). Different species propagated at different rates, the population being built-up during favorable years, became a menace during unfavorable years. A specimen of Melanopius Mexicanus might multiply theoretically in three years to 15,000 while Melanoplus differentialis might produce 500,000 (Ball, 1937)
The migratory locust presented a problem different from the solitary grasshopper. Uvarov (1911, 1921, 1928) elaborated the theory of phase which was based on the assumption that some species were biologically unstable and under certain conditions assumed the solitary form (grasshopper) and under other conditions changed character so completely as to appear to be another species, the migratory, swarming phase (locusts). This theory of phase was originated in connection with Asiatic grasshoppers, but has been applied to the American species Melanoplus Mexicanus Atlantis (synonym Mexicanus Mexicanus), the solitary non-migratory grasshopper. and to the Melanoplus spretus, the swarming Rocky Mountain locust so spectacular in 1874. At any rate M. spretus is practically extinct, never having appeared again in numbers after the seventies, but entomologists warn that the swarming phase might reappear (Parker, 1925; Hebard, 1925; Uvarov, 1928).
On first contact with the grassland, the forest man was impressed by the number of grasshoppers. They were numerous both as to species and numbers. What appeared to him as abnormal, however, was only natural. Hebard (1931) had listed 197 species, 12 subspecies, and no migratory phase found in Kansas. The grasshopper was a typical insect of the grassland and a product of the normal working of biological forces. Every natural region possessed characteristics favorable and unfavorable to the several forms of life, but with an overall long-time tendency to establish and maintain a balance. Grasshopper outbreaks of serious character were of different kinds and were only an exhibition of temporary upsetting of the prevailing unstable equilibrium. Other natural regions with different vegetation and animal life each had their share of pest problems resulting from temporary disturbances of equilibrium, and some pests, especially imported types, sometimes become a permanent threat to forest as well as to grassland regions.
American zoologists discussed less fully the factors which determine the distribution of animals than the botanists had done for vegetation. Merriam's (1890, 1892, 1894, 1898) temperature zone hypothesis received much support among zoologist. The early challengers were Adams (1902) whose work on distribution centers of flora and fauna ran contrary to the life zone maps, and Transeau's (1905) work on climate centers in relation to vegetation was even more explicit. The direct critical analysis and fuller demonstration came later (Shelford, 1911, 1932; Ruthven, 1920; Livingston and Shreve, 1921; Dice, 1923, 1943; Kendeigh, 1932; Smith, 1934; Pitelka, 1941; Hibbard, 1944; summary of the literature, Daubenmire, 1938) and emphasized that there was little correlation when the Merriam transitional zone was applied in detail to particular areas outside those on which Merriam's conclusions were based. The influence of the governmental bureau, The Biological Survey, of which Merriam was head, tended, however, to perpetuate his point of view even after it was rather generally rejected. Revision and restatement of the hypothesis was made by Hall and Grinnell (1919), and Grinnell (1935), in which a rather wide allowance was made for variations under the influence of factors operating locally. It was on the Pacific coast and in the more rugged mountain in areas where temperature changes were relatively abrupt, in respect to space and time that Merriam's hypothesis had its greatest following. Under such special conditions the single factor of temperature might prove to be the principal limiting factor under the law of minimum, especially with the benefit of the allowances recognized by Hall and Grinnell. This would not prove, however, the general validity of the system, or its applicability to other geographical surroundings. On the non-mountainous, or relatively level grassland sound physiological conclusions required a broader base which recognized water, light and soil relations as independent variables. The plants or animals that survived were those that escaped the fatal effects of the extremes but not necessarily the same one, or combinations of two or more of the life factors, depending upon the range of circumstances.
Botanists tended to follow more generally the theory that food controlled animal life, an assumption that subordinated it to vegetation. Walker (1903) emphasized atmospheric moisture, and others sought the solution in hydrogen ion concentration (pH) of the soil. Little attention was given to light (Chapman, 1931), much less among zoologists than among botanists. Allee (1927) rendered a ringing verdict on single-factor procedures in commenting that "There has been a constant chase for a possible single factor index which would be a short cut to environmental analysis. In the very nature of the case, this hunt has met with the failure it deserves." Shelford (1913) had urged as early as 1912 that plant and animal life must be studied together as a unity, and Vestal (1914) developed and demonstrated the idea in his study of Illinois animals. Later ecological work tended rather generally to the broad view that life must be studied as an interacting whole, differences being more in details and theoretical interpretations than in the general hypothesis (Visher, 1916; Allee, 1927; Shelford 1931; Phillips, 1931; Chapman, 1931; Kendeigh, 1934; Clements and Shelford, 1938; Hesse, 1941).
The botanists used the terms plant associations, communities, or assemblages, and the zoologists used similar terms, but in dealing with plants and animals together in the aggregate, new terminology emerged, much of it too complicated for common use, and some of it carrying implications of a subjective rather than strictly objective scientific nature. Shelford (1931) said that "all the life (plant and animal) is a biota"; Clements (1916) offered the term biome, excluding habitat which he viewed as cause with the biome as effect (Clements and Shelford, 1939). Phillips (1931) used the term biotic community. Stemming from the Clement-Shelford terminology, Weese (1941) divided the North American continent into six biomes, with some allowances for areas that did not fit into the system: The Tundra Formation Sedge-Musk Ox Biome); The Coniferous (Evergreen) Forest Formation (Spruce-Moose Biome); The Deciduous Forest Formation (Oak-Deer Biome); The Grassland Formation (Grass-Bison Biome); the Sagebrush Formation (Sagebrush-Jack rabbit Biome); and the Desert Formation (Creosote Bush-Kangaroo Rat Biome). Dice (1943) offered the term biotic provinces and mapped them for North America. The geographers offered physiographic provinces along with maps (Fenneman, 1931; and Atwood, 1940). For the historian none of these terms seems altogether satisfactory for the study of a natural geographical region, but acquaintance with them is essential to the reading of the literature, especially that of the Clements-Shelford school. Among the several difficulties encountered are the implications of uniformity and rigidity which are so far from reality that some zoologists and botanists tend to emphasize variations and the unusual rather than normals and averages, multiple rather than single factors, restating for the purpose Liebig's law of minimum (Chapman, 1931; Taylor, 1934; Rubel, 1935).
The search for physiological explanations of the relations of animals to water, drouth resistance, have been conducted by zoologists, but with less success than that culminating in Maximov's (1929, 1939) treatment of plants. Sumner (1925) doubted whether there were "xerophytic" animals in the sense that there were xerophytic plants. Although desert animals tended to lighter coloration than animals of moist regions, Sumner questioned the adaptational value of such a factor to burrowing, nocturnal animals, and furthermore, why should concealed parts, such as the bottom of the feet show a change in coloration. Dice and Blossom (1927) concluded that there was correlation between animal and soil color. A large number of animals, both mammals and insects escape, rather than resist climatic extremes, by burrowing in the ground or hiding during hot weather, and by burrowing and hibernation during the winter (Sumner , 1925). In Riley county, Kansas, Hayes (1927) found that insects had burrowed as deep as thirty inches in the soil for the winter, coming out again in the spring, a process by which they escaped cold. On the main issue of drouth resistance, it is evident that generalization awaits fuller knowledge.
The influence of light upon animal behavior, in the form of length of day, received its first experimental treatment from Marcovitch in 1923. Marcovitch, Eifrig (1924), and Rowan (1926) and others received their inspiration from the pioneer work of Warner and Allard on photoperiodism in plants. Marcovitch demonstrated that the reproductive development of plant lice was controlled by the shortening of the length of day in Tennessee. Rowan attributed the migration of birds to physiological changes in reproductive organs induced by the factor of change in day length. He pointed out also that chickens increased egg production with the spring change in length of day. The work of Brissonnette (1930-) opened the period of comprehensive studies of light in this relationship.
Park (1940) made the initial attempt to coordinate the accumulation of literature on nocturnalism in animals and to trace the development of the problem for the benefit of the ecologist. It is important to know what animal activities took place in the dark and their relations to the biological equilibrium. About 1919 a new era was opened in the study of the relations of ultra-violet light to rickets in children. Vitamin D deficiency was more than a dietary problem, light being the more fundamental controlling factor. This introduced the problem of color, pigmentation of skin in or hair of animals in relation to light The effect of sunlight in killing bacteria and other microorganisms had long been known, but exact analysis and application were slow. The physical basis of solar radiation, the interruption by ozone, the differences in the effects of the extreme end of the spectrum, infra-red and ultra-violet, daily variation, seasonal variation, differences in latitude, all became biologically important to the distribution of life on the earth (O'Brien, 1943). In 1929, the Smithsonian Institution created a new Division of Radiation and Organisms to deal with the relations of light to living organisms (Annual Report 1929 - )