IN THE THEOEY OF NATURAL SELECTION. 257 



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 mole- 

 cular 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 never- 

 theless 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 hypo- 

 thesis 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 trans- 

 formation arising and working within the organism ; and I must 

 also explain the reasons which induce me to adhere to the theory of 

 natural selection. 



The supposition of such a phyletic force of transformation (see 

 Appendix I, p. 298) possesses, in my opinion, the greatest defect that 

 any theory can have, it does not explain the phenomena. I do not 

 mean to imply that it is incapable of rendering certain subordinate 

 phenomena intelligible, but that it leaves a larger number of facts 

 entirely unexplained. It does not afford any explanation of the 

 purposefulness seen in organisms : and this is just the main problem 

 which the organic world offers for our solution. That species are, 

 from time to time, transformed into new ones might perhaps be 

 understood by means of an internal transforming force, but that 



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