12 JOEL ASAPH ALLEN— CHAPMAN tMM0 ™ vouxxf. 



He then presents in detail the evidence on which his conclusions are based, including a 

 treatment of the regions and provinces contained in his realms, and after a general summary of 

 the data supporting the behef that temperature is the most potent factor governing the dis- 

 tribution of hfe adds: 



Hence, given: 1. Arctic and cold-temperate conditions of climate, and we have a fauna only slightly or 

 moderately diversified; 2. A moderate increase of temperature, giving warm-temperate conditions of climate, 

 and we have the addition of many new types of life; 3. A high increase of temperature, giving tropical condi- 

 tions of climate, and we have a rapid multiplication of new forms and a maximum of differentiation. Again, 

 given: 1. A long-continued continuity of land surface, and we have an essential identity of fauna; 2. A diver- 

 gence and partial isolation of land-areas, and we find a moderate but decided differentiation of faunae; 3. A 

 total isolation of land-areas, and we have a thorough and radical differentiation of faunae, proportioned to the 

 length of time the isolation has continued. Hence, the present diversity of life is correlated with two funda- 

 mental conditions: 1. Continuity or isolation, past as well as present, of land surface; and, 2. Climatic con- 

 ditions, as determined mainly by temperature. 3 



In accordance with these principles, which rest on incontrovertible facts of distribution, it follows that 

 the nearly united lands of the North present a continuous, almost homogeneous, arctopolitan fauna; that farther 

 southward, in the warmer temperate latitudes, we begin to find a marked differentiation on the two continents; 

 that this differentiation is still further developed in the tropical continuations of these same land-areas, till an 

 almost total want of resemblance is reached, except that there is what may be termed, in contrast with the more 

 northern regions, a "tropical fades" common to the two. The small amount of land surface belonging to 

 these primary land regions south of the tropics have no more in common (a few marine species excepted) than 

 have these two tropical areas, but it is hardly possible for them to have much less. The Antarctic (mainly 

 oceanic) region has a fauna strongly recalling the marine fauna of the Arctic, but has no resemblance to that of 

 the intervening area. 



The northern circumpolar lands may be looked upon as the base or centre from which have spread all the 

 more recently developed forms of mammalian life, as it is still the bond that unites the whole. 



Subsequently published papers on distribution treated in detail of the mammals and birds 

 of North America and were in the main elaborations of his earlier contributions to zoogeography. 



It was natural that Doctor Allen 's faunistic studies should arouse his interest in the closely 

 allied subject of migration and, in 1880, he published a brief paper on the " Origin of the instinct 

 of migration in birds, " in which he developed the theory that the seasonal movements of birds 

 are, primarily, due to climatic changes occasioned by glaciation. He said: 



Nothing is doubtless more thoroughly established than that a warm-temperate or subtropical climate 

 prevailed down to the close of the Tertiary epoch, nearly to the Northern Pole, and that climate was previously 

 everywhere so far equable that the necessity of migration can hardly be supposed to have existed. With the 

 later refrigeration of the Northern regions, bird life must have been crowded thence toward the tropics, and 

 the struggle for life thereby greatly intensified. The less yielding forms may have become extinct; those less 

 sensitive to climatic change would seek to extend the boundaries of their range by a slight removal northward 

 during the milder intervals of summer, only, however, to be forced back again by the recurrence of winter. 

 Such migration must have been at first "incipient and gradual," extending and strengthening as the cold wave 

 receded and opened up a wider area within which existence in summer became possible. What was at first a 

 forced migration would become habitual, and through the heredity of habit give rise to that wonderful faculty 

 we term the instinct of migration. 



The explanation here offered of the origin of bird migration remains to-day an accepted 

 theory among students of this phenomenon. 



While prosecuting these more philosophical researches, Doctor Allen was also devoting 

 much time to the production of his classic memoir on The American Bisons, Living and Extinct, 

 a quarto of some 250 pages, published in 1876, to his monographs of North American Rodentia 

 (with Elliot Coues), and to his notable History of North American Pinnipeds, an octavo of 

 some 800 pages which appeared in 1880. 



After accepting the post of curator of birds and mammals in the American Museum of 

 Natural History, the character of Doctor Allen's work of necessity changed. Curatorial, 

 administrative, and editorial duties now demanded all of his time and strength. His thought 



J In illustration of the above, it may be added that the circumpolar lands north ol the mean annual of 36° F., or, in general terms, north of the 

 fiftieth parallel, with approximately an area of about 12,500,000 square miles, have representatives of about 54 genera of mammals; tropical America, 

 with an approximate area of about 5,000,000 square miles, has about 90 genera; the Indo-African Realm, with an approximate area of about 15,000,000 

 square miles, has about 250 genera. Hence the tropical lands are four to five times richer in genera, in proportion to area, than those of the cold- 

 temperate and Arctic regions. 



