THE IMPORTANCE OF EMBRYOLOGY. 121 



present chapter and the next are somewhat differ- 

 ent in their nature from those already considered, 

 and bear more directly on the question of genetic 

 descent. 



When the " Origin of Species " was written, its au- 

 thor did not realize the great aid his views were to 

 receive from the study of embryology, and the sub- 

 ject was passed over by him in a manner which 

 to the scientists of to-day seems entirely unsatis- 

 factory for such an important subject. But this 

 source of evidence did not exist at the time the 

 " Origin of Species " was written, for embryology as 

 a science is of later date. A few embryological facts 

 had at that time been collected, and the generaliza- 

 tions of Von Baer and Rathke had laid the founda- 

 tion of what has since become a science. It was im- 

 possible to appreciate the significance of the facts 

 until evolution was sought as an interpretation. As 

 soon as this theory began to be seriously discussed, 

 it became evident that along the line of embryology 

 might be expected the most cogent arguments upon 

 the question. 



Nor has this expectation proved delusive, for the 

 evidence here afforded has been perhaps the most 

 important factor in leading modern scientists to ac- 

 cept evolution. Although embryology was only 

 slightly considered by Darwin, either in the earlier 

 or later additions of his book, the subject soon ob- 

 tained able exponents. Spencer and Haeckel, with 

 an immediate perception of the significance of the 

 subject, treated it ensemble, and deduced from the 

 then imperfectly known facts some brilliant gener- 



