EVOLUTION 467 



we believe that they are descended from common ancestors. 

 An attempt to classify existing plants and animals leads to an 

 arrangement in which similar plants (or animals) are grouped 

 together into species, or kinds, and these species are grouped 

 together into large assemblages, and these into still larger, and 

 so on, on the basis of resemblances. Now, since we assume 

 relationship or common ancestors in proportion to similarities 

 of structure, 1 the classification suggests that if we go back far 

 enough, we shall find that all birds are related (that is, 

 descended from the same ancestors), and that if we go back 

 still farther, we may find that birds and reptiles are all 

 descended from common ancestors ; or, if we go back still 

 farther, we may find that all backboned animals are descended 

 from the same ancestors. 



We can find no reasonable explanation for this " branching- 

 tree arrangement " of the different kinds of living beings, 

 except the supposition that they have descended from common 

 ancestors and have become modified in the course of time. 



490. Evidence from development. In our study of develop- 

 ment (p. 277) we saw that in the course of each individual's 

 lifetime he passes through a series of more or less distinct 

 stages ; and the farther back we go to the one-celled stage, the 

 more and more are these stages like the corresponding stages 

 of other species of organisms. Moreover, it has been pointed 

 out that the similarities found between different species in the 

 various early stages of development are in a measure parallel 

 to the similarities of the adults or the groups. For example, 

 the larvae of different kinds of mosquitoes are more alike than 

 are the larvae of mosquitoes and beetles ; the larvae of insects 

 in general are more alike than the larvae of insects and crabs ; 

 and so on. And in the life history of a mammal there are 



1 Most of our classification is necessarily based upon structure. A com- 

 parative study of the structure of organisms the branch of science known 

 as morphology shows us similarities in detail of structure that are even 

 more remarkable than the superficial resemblances that are obvious to the 

 casual observer. 



