EVIDENCES OF EVOLUTION IO7 



exhibit some measure of changefulness from generation to 

 generation. This is especially the case if one section of a 

 species be in any way isolated from the rest, or if the animals 

 be subjected in the course of their wanderings to novel 

 conditions of life. 



The evidence from domesticated animals is very convinc- 

 ing. By careful interbreeding of varieties which pleased his 

 fancy or suited his purpose, man has produced numerous 

 breeds of horses, cattle, sheep, and dogs, which are often 

 distinguished from one another by structural differences 

 more profound than those which separate two natural 

 species. In great measure, however, domestic breeds are 

 fertile with one another, while different species rarely are. 

 The numerous and very diverse breeds of domestic pigeons, 

 which are all derived from the rock-dove {Columba livia), 

 vividly illustrate the plasticity or variability of organisms. 



It sometimes happens that offspring resemble not so 

 much the parent as some other form believed or known 

 to be ancestral. Thus a pigeon like the known ancestor 

 Columba livia may be hatched in the dovecot, and a few 

 instances are known of similar reversions to a presumed 

 ancestor. 



{c) Historical. — Among the extinct animals disentombed 

 from the rocks, many form series by which those now 

 existing can be linked back to simpler ancestors. Thus 

 the ancient history of horses, crocodiles, and cuttle-fish is 

 known with a degree of completeness which makes it almost 

 certain that the simpler extinct forms were in reality the 

 ancestors of those which now live. Moreover, many con- 

 necting links have been discovered in the rocks, and the 

 higher animals appear gradually in successive periods of 

 the earth's history 



The facts of geographical distribution, and the history of 

 the diffusion of animals from centres where the presumed 

 ancestral forms are or were most at home, favour the 

 doctrine of descent. 



The individual life-history of an animal — often strangely 

 circuitous or indirect — is interpretable as a modified re- 

 capitulation of the probable history of the race. 



Such, in merest outline, is the nature of the evidence 

 which leads us to conclude that the various forms of life 



