460 LECTURE XVI. 



impassable. Thus we see that the gulf between fishes and rep- 

 tiles has been filled up ; that that between reptiles and birds is 

 beginning to fill ; that some points have been gained to support 

 a bridge over the gulf between reptiles and mammals, the 

 more so as in all vertebrate animals we perceive a unity of 

 structure and a similarity in the fundamental plan, manifesting 

 itself in the unfolding of forms, as well as in the development 

 of the stages, which the embryos of the higher animals have to 

 pass through from the beginning of their existence until they 

 arrive at maturity. But from the Vertebrata to the Inverte- 

 brata I can find no guide, nor have I any idea by what adapta- 

 tion or intermixture intermediate forms can arise, which may 

 lead from the Mollusca and Articulata to the Vertebrata, It is 

 moreover well known that the lowest vertebrate we are ac- 

 quainted with, — the Amphioxiis lanceolatus, is, as regards the 

 development of all its organs, so far behind that of the higher 

 Mollusca and Articulata, that the transition from one of these 

 better developed types into that of this vertebrate would include 

 a series of retrogressions, from which nevertheless is said to 

 have issued the beginning of a structure capable of the highest 

 development. In other words, I see here the vertebrate type, 

 with man as its highest development, commencing with an 

 animal, which, as regards the perfection of its organs, is 

 excelled by most worms, and much more so by most Mollusca 

 and Articulata, which in some instances attain the highest 

 development of which the structural plan of the Articulata is 

 capable. I should thus find myself face to face with an inso- 

 luble enigma, if I were not permitted to recur to the conclu- 

 sion I have arrived at, namely, the assumption of an original 

 difierence in the primary germs from which the animal kingdom 

 has been developed. 



On following the animal structure in its downward direction, 

 we certainly find that the Articulata, step by step, reach the 

 worms, and these again approach the Infusoria so closely, that 

 some naturalists include the latter in the former. On the other 

 hand, the Mollusca approach the Eadiata, there being some 

 forms which naturalists include in either of the above divisions, 

 so that even here an original difierence shews itself. These 



