348 FOUNDATIONS OF BIOLOGY 



gametophyte from the lower to the highest plants; and 

 so on. 



In general, such is the nature of the data which support 

 the evolution theory. Although the evidence, from the 

 nature of the case, must be indirect, it is none the less cogent 

 chiefly because the facts for evolution are from such diverse 

 sources and all converge toward the same conclusion. The 

 theory of evolution reaches the highest degree of probability, 

 since in every branch of botany and zoology all the data 

 are most simply and reasonably explained on the basis of 

 'descent with modification/ and not a single fact points to- 

 ward special creation. It is a cardinal principle of science 

 to accept the simplest conceptions which will embrace all 

 the facts. 



We may now summarize some of the most striking evi- 

 dence from taxonomy, comparative anatomy, paleontol- 

 ogy, embryology, physiology, and distribution of ani- 

 mals. But, as will soon appear, it is impossible to arrange 

 the facts in hard and fast groups under these headings 

 the evidence from one merges into that from another, 

 and in the final analysis nearly all are based on compara- 

 tive anatomy in the broadest sense of the term. 



1. Taxonomy 



When the serious study of classification was well under 

 way, biologists found increasing evidence of the similarity, 

 or affinity, of various SPECIES of animals and plants. Not 

 only is it possible to arrange animals, for example, in an 

 ascending series of increasingly complex forms, but also in 

 many cases it is difficult or impossible to decide just where 

 one species ends and the next begins. That is, the most aber- 

 rant individuals within a given species frequently approach 

 those of a closely similar species. There are intergrades. 



