EVIDENCES OF EVOLUTION. 83 



intelligible. They are relics of past history, comparable, as 

 Darwin said, to the unpronounced letters in many words. 



{b) Physiological. — Observation shows that animals are to 

 some extent plastic. In natural conditions they vary in 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 subjected 

 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 purposes, 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 

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

 the ancestral Columba livia may be hatched in the dove cot, 

 a foal may appear with zebra-like stripes, and in times of 

 famine children may be born who are in some ways ape-like. 

 Such atavisms or reversions are not readily intelligible except 

 on the theory of descent. 



{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 cuttlefish 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 



