372 FOUNDATIONS OF BIOLOGY 



volcanic action alone or aided by coral growth. In either 

 event the organisms are isolated from the main stock of the 

 species, and in proportion to the length of time and the degree 

 of isolation the insular forms diverge until separate races and 

 species arise. Each species peculiar to each isolated island 

 has not arisen by a special act of creation but by descent with 

 modification. 



B. FACTORS OF ORGANIC EVOLUTION 



We have now summarized a few concrete examples of the 

 chief types of evidence that organisms species have 

 come to be what they are to-day through a long process of 

 descent with modification. This evidence, taken with that 

 presented, so to speak, on and between the lines throughout 

 this work, should- place the reader in a position to form a more 

 or less independent judgment of the question. It is only nec- 

 essary to remind him again that, since the evidence, from the 

 nature of the case, must inevitably be indirect, its cogency is 

 tremendously increased by its amount. And the overwhelm- 

 ing impressiveness of all the concordant evidence for organic 

 evolution the reader, with only a very limited amount of 

 the data before him, cannot appreciate. 



Taking for granted the fact of evolution as we have had 

 to do throughout what are the factors which have brought 

 evolution about? That is quite a different question, but one 

 which has often brought confusion to the popular mind. 

 Biologists are not so sure to-day as they were a generation 

 ago that they know just what the factors are. And the lay- 

 man has mistaken their questioning of one factor or another 

 for a questioning of the fact. 



No purpose will be served by a long historical account of 

 the origin of the present-day point of view. Suffice it to say 

 that the evolution idea is a generalization which has crept 



