160 EVOLUTION 



possibly survive, 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 

 with reference to these variations, then the 

 possessors of the fitter variations are bound 

 to be favoured with longer life and larger 

 families — with survival, in short. If this is 

 kept up consistently, then new adaptations 

 and, probably with the help of some form of 

 isolation, new species, will arise. 



The Case for Natural Selection. — The 



