v.] IN THE THEORY OF NATURAL SELECTION. 263 



formation reside within the organism alone. Nageh* further 

 assumes that organisms contain forces which cause periodica! 

 transformation of the species, and he imagines that the organic 

 world, as a whole, has arisen in a manner similar to that in 

 which a single individual arises. 



Just as a seed produces a certain plant because it possesses 

 a certain constitution, and just as, in this process, certain con- 

 ditions must be favourable (light, warmth, moisture, &c.) in 

 order that development may take place, although they do not 

 determine the kind or the manner of development; so, in 

 precisely the same way, the tree of the whole organic world 

 has grown up from the first and lowest forms of life on our 

 planet, under a necessity arising from within, and on the whole 

 independently of external influences. According to Nageli, the 

 cause which compels every form of living substance to change, 

 from time to time, in the course of its secular growth, and 

 which moulds it afresh into new species, must lie within the 

 organic substance itself, and must depend upon its molecular 

 structure. 



It is with sincere admiration and real pleasure that we read 

 the exposition in which Nageli gives, as it were, the result of 

 all his researches which bear upon the great question of the 

 development of the organic world. But although we derive- 

 true enjoyment from the contemplation of the elaborate and 

 ingeniously wrought-out theoretical conception, — which like a 

 beautiful building or a work of art is complete in itself,— and 

 although we must be convinced that its rise has depended 

 upon the progress of knowledge, and that by its means we 

 shall eventually reach a fuller knowledge ; it is nevertheless 

 true that we cannot accept the author's fundamental hypothesis. 

 I at least believe that I am not alone in this respect, and that 

 but few zoologists will be found who can adopt the hypothesis 

 which forms the foundation of Nageli's theory. 



It is not my intention at present to justify my own widely 

 different views, but the subject of this lecture compels me to 

 briefly explain my position in relation to Nageli, and to give 

 some of the reasons why I cannot accept his theory of an active 

 force of transformation arising and working within the organism ; 

 and I must also explain the reasons which induce me to adhere 

 to the theory of natural selection. 



