220 Discovery of the Laws of Evolution.’ [ April, 
that is, acceleration affecting one organ or part more than another, 
thus disturbing the combination of characters which is necessary 
for the state of exact parallelism between the perfect stage of one 
animal and the transitional stages of another. Moreover, acceler- 
ation implies constant addition to the parts of an animal, while 
retardation implies continual subtraction from its characters, or 
atrophy. The speaker had also shown (Method of Creation, 
1871) that the additions appeared either as exact repetitions of 
preéxistent parts, or as modified repetitions, the former resulting 
in simple, the latter in complex organisms. 
Professor Haeckel, of Jena, has added the key-stone to the doc- 
trine of evolution in his gastræa theory. Prior to this generali- 
zation, it had been impossible to determine the true relation ex- 
isting between the four types of embryonic growth, or to speak 
otherwise than to the effect that they are inherently distinct from 
each other. But Haeckel has happily determined the existence - 
of identical stages of growth or segmentation in all the types of 
eggs, the last of which is the gastrula, and beyond which the, 
identity ceases. Not that the four types of gastrula are without 
difference, but this difference may be accounted for on plain prin- 
ciples. In 1874, Haeckel, in his Anthropogenie, recognized the 
importance of the irregularity of time of appearance of the differ- 
ent characters of animals during the period of growth, as affect- 
ing their permanent structure. While maintaining the view that 
the low forms represent the transitional stages of the higher, he 
proceeds to account for the want of exact correspondence ex- 
hibited by them at the present time by reference to this princi- 
ple. He believes that the relation of parent and descendant has 
been concealed and changed by subsequent modification of the 
order of appearance of characters in growth. To the original, 
simple descent, he applies the term palingenesis ; to the modified 
or later growth, ccenogenesis. The causes of the change from 
palingenesis to coenogenesis he regards as three, namely, accel- 
eration, retardation, and heterotopy. 
It is clear that the two types of growth distinguished by Pro- 
fessor Haeckel are those which had been pointed out by the 
speaker, 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 
speaker, as it was a similar impression that led to the publication 
of The Origin of Genera in 1869, 
Fe RUNS | pen OLNEY EO Cee fen ear eR a 
