ON THE THEORY OF EVOLUTION. 125 



tion into the later embryonic stages, chiefly of the skeleton of the 

 Batrachia, in 1865, and Prof. Hyatt, of Salem, Mass., at the 

 same time made similar studies in the development of the Ammo- 

 nites and Nautili. The results as bearing on the doctrine of evo- 

 lution were published in 1869 (in ^^The Origin of Genera"). It 

 was there pointed out that the most nearly related forms of ani- 

 mals do present a relation of repression and advance, or of perma- 

 nent embryonic and adult type, leaving no doubt that the one is 

 descended from the other. This relation was termed exact paral- 

 lelism. It was also shown that, if the embryonic form were the 

 parent, the advanced descendant was produced by an increased 

 rate of growth, which phenomenon was called acceleration ; but 

 that if the embryonic type were the offspring, then its failure to 

 attain to the condition of the parent is due to the supervention of 

 a slower rate of growth ; to this phenomenon the term retarda- 

 tion was applied. It was then shown that the inexact parallelism 

 was the result of U7iequal acceleration or retardation ; that is, ac- 

 celeration 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 state of another. Moreover, accelera- 

 tion implies constant addition to the parts of an animal, while re- 

 tardation implies continual subtraction from its characters, or 

 atrophy. He had also shown (''Method of Creation," 1871) that 

 the additions either appeared as exact repetitions of pre-existent 

 parts, or as modified repetitions, the former resulting in simple, 

 the latter in more complex organisms. 



Prof. Haeckel, of Jena, has added the keystone to the doc- 

 trine of evolution in his gastrsea theory. Prior to this generaliza- 

 tion it had been impossible to determine the true relation existing 

 between the four types of embryonic growth, or, to speak other- 

 wise, than that they are inherently distinct from each other. 

 But Haeckel has happily determined the existence of identical 

 stages of growth in all of 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 differ- 

 ence maybe accounted for, on plain principles. In 1874, Haeckel, 

 in his "Anthropogenic," recognizes the importance of the irregu- 

 larity of time of appearance of the different characters of animals, 

 during the period of growth, as affecting their permanent struct- 

 ure. While maintaining the view that the low forms represent 



