326 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



to the great diversity met with under tropical latitudes ? The funda- 

 mental question which underlies the whole subject is. Is, or is not, the 

 life of the globe distributed in circumpolar zones I The second is, How 

 and under what influences does it become differentiated? 



To the first of these questions, I ventured some six years since,* to 

 give an affirmative answer, in accordance not only with the views of 

 numerous high authorities on the subject of the geographical distribu- 

 tion of life, but with what seemed to me to be incontrovertibly the facts 

 in the case. While this view has since received the support of other 

 high authorities, it has been altogether ignored by the advocates of Dr. 

 Sclater's division of the earth's surface. Mr. Wallace, who faithfully 

 reflects the views of the Sclaterian school, in referring to this subject 

 says : " Mr. Allen's system of ' realms' founded on climatic zones . . . 

 calls for a few remarks. The author continually refers to the * law of 

 the distribution of life in circumpolar zones ', as if it were one generally 

 accepted and that admits of no dispute. But this supposed Maw' only 

 applies to the smallest details of distribution to the range and increas- 

 ing or decreasing numbers of species as we pass from north to south, or 

 the reverse ; while jt has little bearing on the great features of zoologi- 

 cal geography the limitation of groups of genera and families to cer- 

 tain areas. It is analogous to the 4 law of adaptation* in the organiza- 

 tion of animals, by which members of various groups are suited for an 

 aerial, an aquatic, a desert, or an arboreal life ; are herbivorous, carniv- 

 orous, or insectivorous ; are fitted to live underground, or in fresh waters, 

 or on polar ice. It was once thought that these adaptive peculiarities 

 were suitable foundations for a classification, that whales were fishes, 

 and bats birds ; and even to this day there are naturalists who cannot 

 recognize the essential diversity of structure in such groups as swifts 

 and swallows, sun-birds and humming-birds, under the superficial dis- 

 guise caused by adaptation to a similar mode of life. The application 

 of Mr. Allen's principle leads to equally erroneous results, as may be 

 well seen by considering his separation of Hhe southern third of Aus- 

 tralia ' to unite it with.New Zealand as one of his secondary zoological 

 divisions."! 



Leaving Mr. Wallace's last-quoted objection for notice in another 

 connection (see a foot-note beyond, under the sub-heading " Australian 

 Bealm"), I unblushingly claim, in answer to the main point, that the 

 geographical distribution of life is by necessity in accordance with a " law 

 of adaptation", namely, of climatic adaptation ; that such a law is legiti- 

 mate in this connection, and that the reference to the " superficial dis- 

 guise " adapting essentially widely different organisms to similar modes 

 of life is wholly irrelevant to the point at issue, a comparison of things 

 that are in any true sense incomparable ; furthermore, that the "law of 

 distribution of life in circumpolar zones " does apply as well in a gen- 

 eral sense as to details "to groups of genera and families" as well as 



*Bull. Mus. Comp. Zool., vol. ii, p. 376, 1871. 

 tGeogr. Dist. Anim., vol. i, p. 67. 



