444 LECTURE XVI. 



glad, to tell the truths I miglit be to accept the theory of spon- 

 taneous generation ; however illogical it appears to me to 

 assume for the production of organic beings a special force in 

 nature^ which we observe nowhere else ; however natural it 

 may be to search in this primeval generation for the starting 

 point of organic creation, which might then have developed 

 itself in various directions by the influence of various causes ; 

 I must on the other hand confess, that only the most stringent 

 proofs could induce me to adopt it. If this be forthcoming I 

 shall accept it most willingly ; till then, I must acknowledge 

 the existence of this gap in our science, though I entertain the 

 hope that we shall finally succeed in filling it up. 



Organic creation, considered in its totality, presents a remark- 

 able diversity. The chief kingdoms of nature, the vegetable 

 and animal kingdoms^ appear sharply separated from each 

 other. Even within these kingdoms there are some divisions 

 so dissimilar in structure and fundamental plan, that again we 

 imagine a gulf to exist between them. It is, however, soon 

 perceived that there are some groups which have a closer 

 connection; that the similarities of structure grow out during 

 the development of the individual ; that the groups related to 

 each other proceed from a common fundamental form, from 

 which these diversities very gradually arise. 



It was no small triumph for microscopic science when 

 Schwann proved that aU the tissues, whether vegetable or 

 animal, owe their origin to certain elementary forms which he 

 called cells, — a fact now generally admitted. There exists at 

 present no doubt with regard to the development of every 

 vegetable or animal organism from a single cell, — from the egg. 

 There are organisms, both vegetable and animal, consisting of 

 one cell only, which is endowed with all capacities requisite 

 for life and propagation. All other organisms, however com- 

 plicated they may be, are merely masses of cells, differently 

 shaped and grouped, and have aU been developed from one 

 primordial cell. 



Whilst thus the unity of the fundamental plan in the struc- 

 ture of the vegetable and animal world is no longer doubtful ; 

 whilst it is manifest that there are a number of primitive organ- 



