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

 The causes of the change from palingenesis to coenogenesis, be 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 attaches 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 accelerated 

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

 never be disturbed ; while the inexactitude of the parallelism will 

 depend on the number of variations in the rate of growth of dif- 

 ferent organs of the individual, with additions introduced from 

 time to time. Hence it may be laid down, that synchronous ac- 

 celeration or retardation produces exact parallelism, and hetero- 

 chronous acceleration or retardation, produces inexact parallel- 

 ism. 



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

 tation, the protoplasma or animal portion of the primordial egg, 

 or retardation of segmentation of the deutoplasma or vegetative 

 half of the egg, or both, or the same relation between the growth 

 of the circumference and centre 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 : — 



» § O a S"^ CB § 

 -* c& T3 s^ <a S s 



?* ^> h* ^ ~ C*. ^? 







5 * a & 



acceleration, 



which proceeds by 



! Exact repetition * 

 Modified repetition * 

 Heterotopy * 



retardation, •- ( Exact atrophy * 



which proceeds by ( Inexact atrophy (or senility) 1 . . * 



1 So called by Professor Hyatt. 



