CHAPTER XIII 



EVOLUTION AND THE DISTRIBUTION OF ANIMALS ^ 



In the Nineteenth Century of December 1878, Dr. P. L. 

 Sclater called attention to the subject of the geographical 

 distribution of animals in its bearing on the theory of 

 evolution, and gave numerous special cases in which the 

 actual distribution of particular species and groups is very 

 difficult to explain on that theory without making assump- 

 tions which, in his opinion, the evidence at our disposal 

 does not warrant. Difficulties of this nature are so 

 numerous, and many of them seem to him so weighty, that 

 in order to explain them, he is led to question, what is 

 almost an axiom with evolutionists, that identity of Uruc- 

 ture is, without exception, an indication of descent from a 

 common parent. Similar doubts, though not stated in 

 exactly the same terms, were felt by the late Professor 

 Mivart ^ ; and it therefore becomes a matter of interest to 

 examine a little more closely into the alleged difficulties, 

 in order to see whether they are not really explicable on 

 the principle of descent with modification, only calling to 

 our aid such general assumptions as are fully warranted 

 by what we actually know of the migrations and extinctions 

 of living things, and of the past changes in the physical 

 condition of the earth and its inhabitants. 



As Dr. Sclater's article gives an excellent summary of 

 the nature and meaning of zoological distribution, and of 



1 This chapter first appeared in the Nineteenth Century of February 

 1879, under the title " Animals and their Native Countries." 



2 Genesis of Species, Chap. Ill, 



