Chapter IV 



THE MATERIALS OF EVOLUTION 



The apparent permanence of the types of animals 

 and plants living at the present time is a common 

 fact of observation. If this were the whole story it 

 would appear that evolution had come to an end. 

 If living things at the present time were really 

 stable, we could give no good reason why it has not 

 always been so in the past. On the other hand, if this 

 stability is deceptive, we might expect to find evolu- 

 tion still taking place at the present time as in the 

 past, and if this is true we might hope by a careful 

 study of what is happening in living things about us 

 to be able to get some information as to the way in 

 which the process has taken place in the past. It is, 

 of course, also conceivable that, even if evolution 

 went on in the past, it has actually come to an end at 

 the present time, or at least, having reached its cli- 

 max, a declining process may be the order of the 

 day — a process the reverse of that by which the up- 

 ward trend of evolution went on in the past. It is 

 also conceivable that the process of evolution is so 

 slow that we may not be able to detect or measure it 

 with means at our disposal. It may be true, further- 

 more, that certain species at least have become so 

 far adjusted to the present conditions of the earth, 



