THE ORIGIN OF SPECIES 347 



fore against the idea occasionally advanced by men, as it now 

 appears, ahead of their times, that DESCENT WITH MODIFICA- 

 TION is the explanation of the origin of the diverse forms 

 of plank and animals. But, as nearly every one knows, a 

 complete reversal of opinion has occurred since 1860 to- 

 day professional scientists and most educated laymen 

 accept ORGANIC EVOLUTION. And we have accepted it in the 

 preceding sections of this work; but if this appears to have 

 been prejudging the question, the explanation is that the 

 genetic connection of organisms is the guiding principle of all 

 biology and the mere fact that an unbiased presentation 

 of the data seems to prejudge the question is the most cogent 

 presumptive evidence for evolution. It is true that there are 

 wide differences of opinion among biologists in regard to the 

 factors which have brought about the evolutionary change 

 but there are none in regard to the fact of evolution itself. It 

 will be convenient, therefore, first to summarize the evidences 

 of evolution and then to discuss modern views in regard to 

 the methods of evolution. (See Glossary, 'evolution.') 



A. EVIDENCES OF ORGANIC EVOLUTION 



To one who has thoughtfully followed the preceding pages 

 there must immediately occur many facts which are readily 

 and reasonably interpreted from the point of view of descent 

 of one species from another, but which are entirely enig- 

 matical from that of the special creation of species. For 

 instance, one will recall the cellular structure of all organisms; 

 the method of origin and the fate of the germ layers in ani- 

 mals; the interrelationship of the urinary and reproductive 

 systems in the Vertebrates; the comparative anatomy of 

 the vascular and skeletal systems of Vertebrates; the simi- 

 larity of the physical basis of inheritance in animals and 

 plants; the gradual dominance of the sporophyte over the 



