266 GENERAL BIOLOGY 



the developmental lines traceable backward through both 

 ontogeny and phylogeny are all convergent. They point to 

 a common origin in the remote past, and to "descent with 

 modification." 



III. THE PROCESSES OF EVOLUTION; ATTEMPTED EXPLA- 

 NATIONS. 



Facts, such as have been before us in the preceding studies 

 have satisfied biologists generally that evolution has been 

 the method of nature; but the theories that have been 

 advanced in explanation of the processes whereby evolution 

 has been wrought out, have not met with so general accep- 

 tance. Yet, if evolution has had a past, it will have a 

 future; and that future is of importance to us, because it 

 must include the destiny of all races, including our own. 

 Nothing could be of more practical importance to us than 

 that we should understand the conditions of evolutionary 

 progress, especially if these conditions should prove amen- 

 able to our control. 



Many explanations have been offered, and some of them 

 appear in part really to explain. All of them are under 

 scrutiny at the present time. Investigations are in progress 

 to determine their validity. It is well to reserve judgment, 

 but it is also well to know the main features of the current 

 explanations; for such knowledge is part of the common 

 intelligence. Some of the more important explanations 

 will, therefore, be outlined briefly here and in the next 

 chapter. 



Natural selection. The first explanation to receive any 

 general approval (or even to attract much notice) was that 

 of Charles Darwin. He observed how breeders, by selecting 

 and isolating new forms as they arise in domesticated ani- 

 mals and plants, are able to establish new varieties or 

 races. He saw them producing perfectly definite results; 



