190 Animal Life and Intelligence. 



Many zoologists believe, or until lately have believed, 

 that disuse is itself a factor in the process. Just as the 

 well-exercised muscle is strengthened, so is the neglected 

 muscle rendered weak and flabby. Until recently it was 

 generally held that the effects of such use or disuse are 

 inherited. But now Professor Weismann has taught us, 

 if not to doubt ourselves, at least to admit that doubt is 

 permissible. On the older view, the gradual dwindling of 

 unused parts was readily comprehensible. But now, if Pro- 

 fessor Weismann is right, we must seek another explanation 

 of the facts ; and, in any case, we may be led to recognize 

 other factors (than that of disuse alone) in the process. 



Professor Weismann regards panmixia, or free inter- 

 crossing, when the preserving influence of natural selection 

 is suspended, as the efficient cause of a reduction or de- 

 terioration in the organ concerned. And Mr. Bomanes 

 had, in England, drawn attention to the fact that the 

 cessation of natural selection " would lead to some 

 dwindling of the organ concerned, since it was no longer 

 kept up to standard. In illustration of his panmixia, Pro- 

 fessor Weismann says, "A goose or duck must possess 

 strong powers of flight in the natural state, but such 

 powers are no longer necessary for obtaining food when it 

 is brought into the poultry-yard, so that a rigid selection 

 of individuals with well-developed wings at once ceases 

 among its descendants. Hence, in the course of genera- 

 tions, a deterioration of the organs of flight must neces- 

 sarily ensue, and the other members and organs of the 

 bird will be sensibly affected." * And, again, " As at each 

 stage of retrogressive transformation individual fluctuations 

 always occur, a continued decline from the original degree 

 of development will inevitably, although very slowly, take 

 place, until the last remnant finally disappears." f Now, 

 I think it can be shown that panmixia, or the cessation of 

 selection, alone cannot affect much reduction. It can only 



* "Weismann, " Essays on Heredity," p. 90. 



t Ibid. p. 292. See also a discussion in Nature, in which Mr. Eomanes 

 and Professor Kay Lankester took part, beginning vol. xli. p. 437. 



