206 DEGENERATION sec. 



an organ as natural selection can cause it to develop. Selec- 

 tion is, I must ever repeat, no physiological factor which 

 could directly produce anything new, or whose cessation 

 could annul anything existing. Organs are produced by 

 external stimuli, or by use acting upon the material given in 

 a given case, with the aid of general and of sexual selection. 

 Weismann, while he refuses to recognise what I assume to 

 be the principal influences as having any important effect, 

 while he puts forward natural selection or its cessation as 

 active accents, brinies forward another factor to aid in the 

 carrying out of the modification due to these agents, namely, 

 sexual intermixture. According to him, the degeneration of 

 the powers of organs and the organs themselves is not due to 

 the diminution and cessation of its activity, with the aid of 

 the cessation of natural selection and of sexual intermixture, 

 but is due to the latter two causes only. He says those parts 

 which are no longer useful are no further subject to selection 

 — the individuals multiply sexually, therefore, equally whether 

 those parts are developed or not. All individuals unite 

 without reference to the character in question, and therefore, 

 through pammixis, through general intermixture, the char- 

 acter must vanish. 



Weismann's explanation of degeneration is certainly 

 correct, so far as the proposition from which it is derived 

 is correct — the proposition that all adaptation of forms 

 depends on natural selection ; and, I must add, so far as all 

 that exists in the forms of organic nature depends on 

 adaptation. 



I have, however, strenuously opposed these propositions, 



of the term natural breeding may easily lead to the error of treating selection 

 as an active force. It would be better always to use the term selection where 

 the survival of the fittest in the struggle for existeuce is meant, since natural 

 breeding, according to the view advocated by me, can occur without any 

 selection. Directly acting external stimuli and the use or disuse of organs effect 

 modifications of living forms, and this may be properly described as natural 

 breeding. 



