"THE ORIGIN OF SPECIES." 325 



conclusions which I then laid before you has since come 

 to light. 



In 1875, the discovery of the toothed birds of the 

 cretaceous formation in North America by Professor 

 Marsh completed the series of transitional forms be- 

 tween birds and reptiles, and removed Mr. Darwin's 

 proposition that " many animal forms of life have been 

 utterly lost, through which the early progenitors of birds 

 were formerly connected with the early progenitors of 

 the other vertebrate classes," from the region of hy- 

 pothesis 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 impor- 

 tant, 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 Tuni- 

 cata prove, beyond a doubt, that the differences 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 man- 

 ner in which the transition was actually effected may 

 still be lacking. 



Again, in 1859, thers 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 inaugurated by Hof- 



