126 GENERAL EVOLUTION. 



the transitionaL stages of the higher, he proceeds to account for 

 the want of exact correspondence exhibited by them at the present 

 time, by reference to this principle. He believes that the rela- 

 tion of parent and descendant has been concealed and changed 

 by subsequent modifications of the order of appearance of charac- 

 ters in growth. To the original, simple descent he applies the 

 term palingenesis ; to the modified and later growth, coenogenesis. 

 The causes of the change from palingenesis to coenogenesis he re- 

 gards as three, viz., acceleration, retardation, and heterotopy. 



It is clear that the two types of growth distinguished by Prof. 

 Haeckel are those which had been pointed out by Prof. Cope in 

 "The Origin of Genera," as producing the relations of "exact" 

 and "inexact parallelism"; and that his explanation of the 

 origin of the latter relation by acceleration or retardation is the 

 same as that of the latter essay. The importance which he at- 

 taches to the subject was a source of gratification to the speaker, 

 as it was a similar impression that led to the publication of " The 

 Origin of Genera " in 1869. 



It remains to observe that the phenomena of exact parallelism 

 or palingenesis are quite as necessarily accounted for on the prin- 

 ciple of acceleration or retardation as are those of inexact paral- 

 lelism or coenogenesis. Were all parts of the organism acceler- 

 ated or retarded at a like rate, the relation of exact parallelism 

 would never be disturbed ; while the inexactitude of the parallel- 

 ism will depend on the number of variations in the rate of growth 

 of different organs of the individual, with additions introduced 

 from time to time. Hence it may be laid down that synchronous 

 acceleration or retardation produces exact parallelism, and lietero- 

 chronous acceleration or retardation produces inexact parallel- 

 ism. 



In conclusion, it may be added that acceleration of the seg- 

 mentation of the protoplasma or animal portion of the primordial 

 Qgg, or retardation of segmentation of the deutoplasma or vegeta- 

 tive half of the Qgg, or both, or the same relation between the 

 growth of the circumference and center of the egg, has given rise 

 to the four types which the segmentation now presents. 



An analysis of the laws of evolution may be tabulated as fol- 

 lows : 



