218 THE PROBLEMS OF EVOLUTION 



force of gravity and a solid substratum. But when 

 the ancestral fishes emerged from the water, the 

 stimulus of a dense fluid medium was lost. Pre- 

 sumably the fringe of the fins disappeared, but we 

 have no reason to suppose that the power to pro- 

 duce a complete fin was lost in these early ter- 

 restrial fishes. They were, however, subject to 

 new stimuli on land, stimuli which favored the 

 realization of latent possibilities of their fins which 

 could never have been brought out by an aquatic 

 environment, and the pentadactyl appendage is 

 the result. In most terrestrial vertebrates its im- 

 portance is so great that reversion is unthinkable; 

 it is a positive adjustment to positive environ- 

 mental conditions, many of which could have had 

 no part in its production, and even though subject 

 to the same aquatic environment as the ancestral 

 fin, it presents a different heritage and so gives a 

 different result in the course of evolution, as in 

 the flippers of the seals. But it can be reduced, like 

 the fore limbs of the kangaroos, or lost completely 

 as in the snakes. 



It seems entirely logical to expect reversibility 

 in the response of a heritage to an environmental 

 stimulus during the initial stages of stimulation, 

 but when the heritage has been subjected to the 

 stimulus for many generations, or to a gradually 

 increasing stimulus, until a cumulative response 

 has been brought to an extreme expression, the 



