COMING OF AGE OF THE ORIGIN OF SPECIES. 341 



In 1868 I had the honor of bringing under your notice, in this 

 theatre, the results of investigations made, up to that time, into the 

 anatomical characters of certain ancient reptiles, which showed the 

 nature of the modifications in virtue of which the type of the quadru- 

 pedal reptile passed into that of the bipedal bird ; and abundant con- 

 firmatory evidence of the justice of the conclusions which I then laid 

 before you has since come to light. 



In 1875 the discovery of the toothed birds of the cretaceous for- 

 mation in North America, by Professor Marsh, completed the series of 

 transitional forms between birds and reptiles, and removed Mr. Dar- 

 win's proposition, that " many animal forms of life have been utterly 

 lost, through which the early progenitors of birds were formerly con- 

 nected with the early progenitors of the other vertebrate classes," from 

 the region of hypothesis to that of demonstrable fact. 



In 1859 there appeared to be a very sharp and clear hiatus between 

 vertebrated and invertebrated animals, not only in their structure, but 

 what was more important, in their development. I do not think that we 

 even yet know the precise links of connection between the two ; but 

 the investigations of Kowalewsky and others upon the development of 

 Amphioxus and of the Tunicata prove beyond a doubt that the differ- 

 ences which were supposed to constitute a barrier between the two are 

 non-existent. There is no longer any difficulty in understanding how 

 the vertebrate type may have arisen from the invertebrate, though the 

 full proof of the manner in which the transition was actually effected 

 may still be lacking. 



Again, in 1859 there appeared to be a no less sharp separation 

 between the two great groups of flowering and flowerless plants. It 

 is only subsequently that the series of remarkable investigations in- 

 augurated by Hofmeister has brought to light the extraordinary and 

 altogether unexpected modifications of the reproductive apparatus in 

 the Lycopodiacece, the Rhizocarpece, and the Gymnospermece, by 

 which the ferns and the mosses are gradually connected with the 

 Phanerogamic division of the vegetable world. 



So, again, it is only since 1859 that we have acquired that wealth 

 of knowledge of the lowest forms of life which demonstrates the 

 futility of any attempt to separate the lowest plants from the lowest 

 animals, and shows that the two kingdoms of living nature have a 

 common borderdand which belongs to both or to neither. 



Thus it will be observed that the whole tendency of biological in- 

 vestigation since 1859 has been in the direction of removing the diffi- 

 culties which the apparent breaks in the series created at that time ; 

 and the recognition of gradation is the first step toward 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 prog- 

 ress which has been made in the study of embryology. Twenty 



