316 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



those long-enduring features of physical geography, on wliicli the distribution 

 of all forms of life fundamentally depends;* and all discrepancies in the 

 distribution of other classes of animals must be capable of being ex- 

 plained, either by their exceptional means of dispersion or by special 

 conditions affecting their perpetuation and increase in each locality." 

 " If these considerations are well founded," he continues, " the objections 

 of those who study insects or molluscs, for example, that our regions are 

 not true for their departments of nature cannot be maintained. For 

 they will find, that a careful consideration of the exceptional means of 

 dispersal and conditions of existence of each group, will explain most 

 of the divergences from the normal distribution of higher animals." t 



In the present paper I shall consequently, in my discussion of the 

 zoological regions of the northern hemisphere, confine myself primarily 

 to mammals. Throwing aside, for the moment, all theoretical consider- 

 ations, I shall endeavor first to present the facts of the case, and then 

 consider what generalizations may be legitimately drawn from them. 



A word, however, first in respect to the conformation and distribu- 

 tion of the land-areas. In reference to this part of the subject I can 

 hardly do better than to again quote the words of Mr. Wallace, who has 

 thus forcibly presented the subject : " One great peculiarity of the dis- 

 tribution of land lies in its freedom from complete isolation . . . The 

 continents, indeed, resembling as they do a huge creeping plant, with 

 roots at the North Pole, and the matted stems and branches of which 

 cover a large part of the northern hemisphere and send three great off- 

 shoots toward the South Pole, offer great facilities for the transmission 

 of varied forms of animal life. There is evidence to prove that during 

 the greater part of the Tertiary period the relative positions of our conti- 



* The italicizing is my own. 



t The question, Which class of animals is best fitted to form the basis of a division 

 of the earth's surface into life-regions ? has a wider bearing than might be at first sup- 

 posed, since the same power of adaptation to diverse climatic conditions that results 

 in a wide distribution in some cases and a limited range hi others would also impart 

 different degrees of ability to resist the influence of geological changes, and is hence 

 related to the question, Which class forms the best index for marking geological time ? 

 The relative importance of different groups as geological indices is necessarily con- 

 nected with their power to resist unfavorable influences, and hence groups that suc- 

 cumb most readily would give the best clue to such changes in the past. Among ver- 

 tebrates the mammalia are undoubtedly, as a class, the best able to survive a wide 

 range of climatic conditions. Birds are to so great a degree migratory that they are 

 in great measure able to avoid seasonal extremes of climate by a change of habitat. 

 Extremes that mammals readily survive prove quickly fatal to reptiles and amphibians. 



Climate, though in itself a powerful geological agent, is, of course, subject to profound 

 modification due to geological causes. Any great amount of upheaval or subsidence 

 of the earth's crust, or the gradual uplifting of mountain chains, must necessarily 

 induce changes in the climate of the regions where such disturbances occur, the effect 

 of which must extend over an area far greater than that of the disturbed district. A 

 comparatively slight change of climate, either in respect to temperature or humidity, 

 has a most marked influence upon vegetation, and especially upon the distribution of 

 forests. The presence or absence of particular species of plants is well known to 

 determine the presence or absence of many species of insects, while the distribution of 

 whole families of the la.tter is determined wholly by the character of the vegetation j 



