RESUME 329 



active reactions of the organism, with the use and 

 disuse of organs; when, however, it considers the di- 

 rect action of climate, temperature or nutrition, it can 

 account for any variations whatever. Adaptation in 

 this case becomes a more obscure and more problem- 

 atic phenomenon. 



Let us first take the problem of adaptation and see 

 whether it has been explained satisfactorily by any 

 of the systems we have passed in review. The crit- 

 icisms formulated against the idea that natural selec- 

 tion fosters infinitesimal individual variations and 

 brings about, without the help of any other factor, the 

 entire phylogenetic development, are so serious and 

 based upon evidence so positive, that no biologist will 

 ever again consider natural selection as the exclusive 

 factor. Natural selection can of course destroy in- 

 jurious variations, especially if they are very pro- 

 nounced, but scientists agree that it cannot foster 

 useful variations. 



The development of useful organs is very easily 

 explained by use which fosters their growth, but, 

 though this theory is self-evident within the limits of 

 the individual's life, it becomes more questionable 

 when we consider the individual's offspring. The 

 offspring cannot be benefited by any favourable re- 

 sult of organic activity in the parent unless this re- 

 sult is transmitted hereditarily. And it is precisely 

 when we consider the characters due to use or disuse, 



