Organic Evolution. 209 



not support the idea that males with the tail-feathers a 

 trifle longer, or the colours a trifle brighter, are generally 

 preferred, and that those which are only a little inferior are 

 as generally rejected, — and this is what is absolutely needed 

 to establish the theory of the development of these plumes 

 by means of the choice of the female." * If Mr. Wallace 

 requires direct observational evidence of this kind, I do not 

 suppose he is likely to get any large body of it. But one 

 might fairly ask him what body of direct observational 

 evidence he has of natural selection. The fact is that 

 direct observational evidence is, from the nature of the 

 processes involved, almost impossible to produce in either 

 case. Natural selection is an explanation of organic 

 phenomena reached by a process of logical inference and 

 justified by its results. It is not claimed for the hypo- 

 thesis of selective mating that it has a higher order of 

 validity. 



Use and Disuse. 



As we have already seen, biologists are divided into two 

 schools, one of which maintains that the effects of use and 

 disuse f have been a potent factor in organic evolution ; 

 the other, that the effects of use and disuse are restricted to 

 the individual. My own opinion is that we have not a 

 sufficient body of carefully sifted evidence to enable us to 

 dogmatize on the subject, one way or the other. But, the 

 position of strict equilibrium being an exceedingly difficult 

 and some would have us believe an undesirable attitude 

 of mind, I may add that I lean to the view that use and 

 disuse, if persistent and long-continued, take effect, not 

 only on the individual, but also on the species. 



It is scarcely necessary to give examples of the kind of 

 change which, according to the Lamarckian school, are 

 wrought by use and disuse. Any organ persistently used 

 will have a tendency, on this view, to become in successive 



* " Darwinism," p. 286. 



t On the negative character of disuse, see p. 196. 



P 



