160 EVOLUTION 



was said by Darwin, or that there is not still 

 abundant opportunity both for reflection and 

 research in regard to it, we must be clear as 

 to the essential simplicity of the general theory. 

 Darwin himself summed it up in a couple of 

 sentences : " As many more individuals of 

 each species are born than can possibly sur- 

 vive, and as, consequently, there is frequently 

 recurring struggle for existence, it follows 

 that any being, if it vary however slightly 

 in any manner profitable to itself, under the 

 complex and sometimes varying conditions of 

 life, will have a better chance of surviving, 

 and thus be naturally selected. From the 

 strong principle of inheritance any selected 

 variety will tend to propagate its new and 

 modified form." 



There are here three main propositions : 



(1) Variability is a fact of life. Variations 

 are of frequent occurrence, and some of them 

 are certainly transmissible. 



(2) Living creatures are very prolific. The 

 majority die young. There is a ceaseless 

 struggle for existence and the web of inter- 

 relations is such that even minute variations 

 may determine survival. 



(3) If variations occur in the direction of 

 increased fitness, if the variations are trans- 

 missible, and if there is discriminate selection 



