86 THE DOCTRINE OF DESCENT, 



the course of several generations, or even in a lifetime. 

 This is especially the case if one section of a species be in 

 any way isolated from the rest, or if the animals be sub 

 jected in the course of their wanderings to novel conditions 

 of life. Even apart from markedly changed circumstances, 

 moreover, animals exhibit variations from generation to 

 generation. 



The evidence from domesticated animals is very con- 

 vincing. 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 the offspring of an animal 

 resemble not so much the parent as some other form be- 

 lieved or known to be ancestral. Thus a blue pigeon like 

 the ancestral Cohtmba livia may be hatched in the dovecot. 

 Such reversions are not readily intelligible except on the 

 theory of descent. 



(f) 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, that many 

 connecting links have been discovered in the rocks, and 

 that the higher animals appear gradually in successive 

 periods of the earth's history, are strong corroborations of 

 the theory. 



It is less easy to state in a few words how the facts of 

 geographical distribution, or 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- 



