412 EVIDENCES OF EVOLUTION 



the camel is a tree whose trunk is the ancestral stock, 

 while the branches represent the evolution of the remote 

 ancestor into different species. The greater part of this 

 tree is dead, the tips of the branches, which correspond 

 to the modern species, being the only living portion. 

 Other families of animals and plants whose ancestral 

 trees have been carefully followed into the earth, and 

 the fragments of those which yearly are brought to light, 

 all illustrate these same general principles. 



In the attempt to classify these living species it was 

 long ago discovered that resemblances and differences of 

 structures are the only safe guide. Shape, size, color, 

 and other superficial characters were used by the biologist 

 a century or more ago, but they were found to be practi- 

 cally worthless. Various schemes of graphically repre- 

 senting relationships were also abandoned in favor of the 

 tree-like branching plan, such as is used to illustrate the 

 genealogy or relationship of various human families. In 

 other words, the earlier scientists to whom evolution 

 was unknown were driven to adopt a system of classifica- 

 tion approximating that of the present day. Their work, 

 however, was little more than an intellectual exercise, a 

 pigeonholing of organisms for convenience only. To-day 

 it remains a convenient system, yet at the same time 

 it expresses the blood relationships of living things, and 

 is one of the strong proofs that evolution has occurred. 



The Modern System of Classification. — According 

 to the present system of classification, a species is defined 

 as an assemblage of similar animals or plants, which do 

 not differ in size, shape, color, etc., beyond the limits of 

 individual variation. Furthermore, these characters are 

 inherited with but slight modification ; and the group is 

 not united with any other by connecting links. Now let 

 this definition be applied to the group of cats. Among 

 the domesticated animals there are many breeds, persian, 

 maltese, manx, etc., but these do not constitute distinct 

 species, since there are many intermediates. They are 



