Heredity and the Origin of Variatio7is. 175. 



to their fattening capacity. Those with longer intestines, 

 and therefore increased absorbent surface, may well have 

 an advantage in this respect. Hence, in selecting pigs for 

 fattening, breeders may have been unconsciously selecting 

 those with the longest intestines. Of course, on this view, 

 the longer intestine must be there to be selected, and the 

 increased length must be due to variation. But this may 

 be all-round variation (cause unknown), not variation in 

 one direction, the result of increased function. 



Another point that has to be taken into consideration- 

 is the amount of individual increment or decrement, owing 

 to individual use or disuse, apart from any possible 

 summation of results. 



Seeing, then, that it is difficult to estimate the amount 

 of purely individual increment or decrement, and that it is 

 difficult, if not impossible, to exclude the disturbing effects 

 of cessation of selection with economy of growth on the 

 one hand, reducing the size of organs, and artificial 

 selection on the other hand, increasing the size or efficiency 

 of parts, it is clear that such cases cannot afford convincing 

 evidence that the observed variations are the directly 

 inherited results of use and disuse. Indeed, I am not 

 aware of any experiments or direct observations on animals- 

 which are individually conclusive in favour of the hereditary 

 summation of functionally produced modifications. 



It may, however, be said — Although no absolutely con- 

 vincing experiments or observations are forthcoming (for,, 

 from the nature of the case, it is almost impossible logically 

 to prove that this interpretation of the facts is alone 

 possible), still there are cases which are much more readily 

 explained on the hypothesis that the effects of use and 

 disuse are inherited, than on any other hypothesis. But,, 

 so far as Professor Weismann and his followers are con- 

 cerned, such an argument is wholly beside the question. 

 They are ready to admit that inherited modifications of the 

 body, if they could be proved, would render the explanation 

 of many results of evolution much easier. It would, no- 

 doubt, they say, be easier to account for the shifting of the: 



