Vii "THE ORIGIN OF SPECIES " 237 



dency of biological investigation, since 1859, has 

 been in the direction of removing the difficulties 

 which the apparent breaks in the series created 

 at that time ; and the recognition of gradation 

 is the first step towards the acceptance of evolu- 

 tion. 



As another great factor in bringing about the 

 change of opinion which has taken place among 

 naturalists, I count the astonishing progress which 

 has been made in the study of embryology. 

 Twenty years ago, not only were we devoid of any 

 accurate knowledge of the mode of development 

 of many groups of animals and plants, but the 

 methods of investigation were rude and imperfect. 

 At the present time, there is no important group 

 of organic beings the development of which has 

 not been carefully studied ; and the modern 

 methods of hardening and section-making enable 

 the embryologist to determine the nature of the 

 process, in each case, with a degree of minuteness 

 and accuracy which is truly astonishing to those 

 whose memories carry them back to the 

 beginnings of modern histology. And the results 

 of these embryological investigations are in com- 

 plete harmony with the requirements of the 

 doctrine of evolution. The first beginnings of all 

 the higher forms of animal life are similar, and 

 however diverse their adult conditions, they start 

 from a common foundation. Moreover, the pro- 

 cess of development of the animal or the plant 



