THE LAWS OF ADAPTATION 159 



fices others; for example, the lateral digits of the foot of the 

 horse are sacrificed for the evolution of the central digit as the 

 animal evolves from tridactylism to monodactylism. These 

 sacrificed parts are never regained; the horse can never regain 

 the tridactyl condition although it may re-enter a habitat 

 zone in which three digits on each foot would serve the pur- 

 poses of locomotion better than one. In this sense chromatin 

 evolution is irreversible. The extinction of vertebrate races 

 has generally been due to the fact that the various types have 

 sacrificed too many characters in their structural and func- 

 tional reactions to a particular life habitat zone. A finely spe- 

 cialized form representing a perfect mechanism in itself which 

 closely interlocks with its physical and living environment 

 reaches a cul-de-sac of structure from which there is no possible 

 emergence by adaptation to a different physical environment 

 or habitat zone. It is these two principles of too close adjust- 

 ment to a single environment and of the non-revival of char- 

 acters once lost by the chromatin which underly the law that 

 the highly specialized and most perfectly adapted types become 

 extinct, while primitive, conservative, and relatively unspe- 

 cialized types invariably become the centres of new adaptive 

 radiations. 



