PHYLOGENY. 173 



of the subdivisions of the great classes, in the preceding 

 section. 



Agassiz and Dana pointed out this fact in taxon- 

 omy, and I expressed it as an evolutionary law under 

 the name of the "Doctrine of the Unspecialized." 

 This describes the fact that the highly developed, or 

 speciahzed types of one geologic period have not been 

 the parents of the types of succeeding periods, but 

 that the descent has been derived from the less spe- 

 cialized of preceding ages. No better example of this 

 law can be found than man himself, who preserves in 

 his general structure the type that was prevalent dur- 

 ing the Eocene period, adding thereto his superior 

 brain-structure. 



The validity of this law is due to the fact that the 

 specialized types of all periods have been generally in- 

 capable of adaptation to the changed conditions which 

 characterized the advent of new periods. Changes of 

 climate and food consequent on disturbances of the 

 earth's crust have rendered existence impossible to 

 many plants and animals, and have rendered life pre- 

 carious to others. Such changes have been often espe- 

 cially severe in their effects on species of large size, 

 which required food in large quantities. The results 

 have been degeneracy or extinction. On the other 

 hand plants and animals of unspecialized habits have 

 survived. For instance, plants not especially restricted 

 to definite soils, temperatures, or degrees of humidity, 

 would survive changes in these respects better than 

 those that have been so restricted. Animals of om- 

 nivorous food-habits would survive where those which 

 required special foods, would die. Species of small 

 size would survive a scarcity of food, while large ones 

 would perish. It is true, as observed by Marsh, that 



