THE METHOD OF EVOLUTION 329 



the marvelous adaptations of structure which we always find in 

 all living things. 



2. Relationship of all Forms. Carrying the theory to its logical 

 conclusion it follows that all the species now on earth, or which 

 have lived there in the past, are descended from a few primitive 

 original forms. The further back the variation began, the greater 

 will be the difference between the present forms, and the more 

 distant will be then* relationship. Those more closely allied have 

 separated from a common ancestor in more recent times. 



3. " Tree " Lines of Descent. Evidently our idea of the lines 

 of relationship and descent must be expressed in the figure of a 

 tree, whose main branches separated from the parent trunk early 

 in development and whose topmost twigs represent the present 

 living forms. These will be similar or different, depending on how 

 far back the divergence began. 



4. Classification. Evolution provides for a natural method of 

 classification, now universally used, in which relationship and 

 descent are shown by the groups in which individuals are placed. 



Thus members of a species are more closely related than those 

 of a genus or order. A class includes forms which began to diverge 

 further back than the members of a family. When we speak of any 

 forms as " belonging to the same order " or genus, we are really 

 expressing not only their likeness in structure, but the reason for 

 it, namely blood relationship and descent from common ancestry. 



5. The Key to other Biologic Puzzles. Evolution accounts for 

 many facts otherwise unexplained. It tells us why we find fossil ! 

 remains of simpler animals in older rocks, and of more highly 

 specialized forms in later formations. It accounts for the facts 

 of embryology mentioned in the previous chapter, such as the 

 occurrence of primitive structures in the embryos of higher forms, 

 which disappear before maturity. It explains the peculiarities of 

 geographic distribution of animals and plants, in accordance with 

 what we know of past and present relations of land and sea areas. 



Some Things that Evolution does Not Teach. 1. That living 

 or extinct forms can be arranged in a straight line of descent, each ; 

 descended from its predecessor. 



