CHAPTER XVII. 



THE PLASTICITY AND THE PERMANENCY OF CHARAC- 

 TERS IN THE HISTORY OF ORGANISMS. 



Races in Paleontology — During the life-period of a genus 

 constant changes are found to take place among the represent- 

 atives of the genus as we follow them upward from stage to 

 stage of their geological succession. The forms appearing at 

 the first epoch, in the life-period of a genus, are generally 

 found to be of different species from those occurring later; 

 and in many genera there are enough specimens collected, and 

 sufificient knowledge regarding them accumulated, to enable 

 the paleontologist to recognize a series of forms regularly 

 succeeding one the other, presenting slight modification from 

 one stage to the next, but those of each stage showing closer 

 resemblance to those immediately preceding them than to any 

 other species of the same genus. The series of forms thus 

 resembling each other may be called races, because of the 

 very evident genetic relationship existing between the later 

 and the earlier representatives of the series. 



Phylogeny of the Race. — When we examine the details of 

 form in such a series of succeeding forms or races of a genus, 

 comparatively, it is often apparent that the changes under- 

 gone in respect to each character are progressive or of an 

 accumulative nature, and thus they resemble the changes 

 which the individual undergoes in ordinary growth. The 

 technical name proposed by Haeckel for this morphological 

 history of the race is PJiylogeny, contrasting it with Ontogeny 

 or the history of growth or development of the individual, 

 from its relatively homogeneous condition in the ovum to the 

 more or less differentiated adult organism. 



Mutability and Phylogeny. — The Cuvierlan school of natu- 

 ralists believed in the immutability of species, and for them 



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