294 Habit and Instinct. 



water, but received such a vigorous peck on his nose, 

 followed by a second which narrowly missed his eye, that 

 he beat a hasty retreat. I am not aware of any well- 

 substantiated evidence which has been published since 

 Eomanes wrote,* and am forced to conclude that though il 

 must not be summarily set aside, that which we at presenl 

 have is surprisingly small in amount, and peculiar ii 

 distribution, if transmission be the predominant factor in 

 organic evolution. There is, however, a growing con- 

 viction that to solve some of its most pressing problems 

 biologists must pass from the observational to the experi- 

 mental stage. Perhaps experiment will show, more con- 

 clusively than has yet been shown, that transmission is 

 a fact, and will afford the means of estimating its strength 

 in the evolution of the world of living things. 



There is, however, another, though less direct means 

 of approaching the problem. Let us fix our attention 

 on any two individuals of the same species : the one 

 survives and mates ; the other either dies or fails to mate. 

 Now, the former survives in virtue of the possession of 

 some excellence which the other does not possess. It is 

 this excellence, of whatever kind it may be, that deter- 

 mines the survival on the one hand, or elimination on 

 the other ; and it obviously must reach a certain amount 

 termed by Eomanes " selection- value " in order to 

 determine which individual will survive and which suc- 

 cumb. What the survival-value must be in any particular 

 case we do not know it varies no doubt with the severity 

 of the struggle ; but to say, as some do, that an infini- 

 tesimal advantage suffices to differentiate the one from 

 the other, appears to be an assumption based rather on 

 logical than biological grounds. 



Now, natural selectionists fall into two wings. Those 



* The evidence adduced by Prof. Eimer fails to carry conviction. 



