PLASTICITY AND PERMANENCY OF CHARACTERS. 295. 



the principle of racial evolution or phylogeny was barred out. 

 But Geoff roy St. Hilaire and Lamarck with their idea of mu- 

 tability of species laid the way for a consistent theory of 

 phylogenetic evolution, although in their time the knowl- 

 edge of paleontology was not far enough advanced to furnish 

 actual phylogenetic series of organisms. It was, however, 

 not till Darwin had constructed a working hypothesis for 

 the steps and manner by which new types of organisms can 

 arise, that evolution became an accepted mode of explana- 

 tion of the course of biological history. 



The great advance which the present generation has wit- 

 nessed in the interpretation of the science of organisms is the 

 change in belief, which all naturalists have more or less thor- 

 oughly undergone, from the doctrine of immutability to that 

 of mutability of species. Some theory of evolution and 

 phylogenetic origin of species is the necessary outcome of 

 this new doctrine. Darwin more than any other single man 

 was the means of producing the change of conviction in re- 

 gard to this point. 



The Phylogenetic Theory of Evolution. — The phylogenetic 

 theory of evolution is logically an expansion and application 

 of the principle of organic growth, already recognized in the 

 development of individual characters, to the evolution of spe- 

 cific and more fundamental differences. It is a recognition 

 of an organic correlation between separate individuals. As 

 growth takes place in the individual by the segmentation 

 and separation of cells, with specialization of functions, first 

 for different cells and finally for the complex structures 

 called organs, the whole showing its organic unity by the 

 mutual cooperation of all of the parts in the life of the 

 whole, so the phylogenetic theory recognizes in the species, 

 or the race of species, an organic unity of a higher sphere, in 

 which the individuals play the part of mutually adjusted and 

 cooperating parts in this greater organic whole. 



The theory goes one step further, and includes the propo- 

 sition, that as the principle onine viviun ex ovc is true in the 

 life-history of individuals, so each species postulates a pre- 

 existing species. This is the philosophy of the theory, but 

 it must be observed that the concrete facts illustrating- these 



