EVIDENCES OF EVOLUTION. 8r 



theory that they are vestiges of structures which were of 

 use in ancestors are these rudiments inteUigible. They are 

 relics of past history like 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 notably changed circumstances, 

 moreover, animals exhibit variations from generation to 

 generation. 



(2) The evidence from domesticated animals is very pre- 

 cise and convincing. 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 livid), vividly illustrate the plasticity or 

 variability of organisms. 



(3) 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 rnay 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. — (i) Among the extinct animals disen- 

 tombed 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 



