xn. ] "THE ORIGIN OF SPECIES." 319 



and shows that the two kingdoms of living nature 

 have a common borderland which belongs to both or 

 to neither. 



Thus it will be observed that the whole tendency 

 of biological investigation, since 1859, has been in the 

 direction of removing the difficulties which the appa- 

 rent breaks in the series created at that time ; and the 

 recognition of gradation is the first step towards the 

 acceptance of evolution. 



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 in- 

 vestigation 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 complete 

 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 process of development 



