The speaker, not adopting either view, made a full investigation 

 into the later embryonic stages, chiefly of the skeleton of the Ba- 

 ti'achia, in 1865, and Prof. Etyatt, of Salem, Mass., at the same 

 time made similar studies in the development of the Ammonites 

 and Nautili. The results as bearing on the doctrine of evolution 

 were published in 1869 (in " The Origin of Genera"). It was there 

 pointed out, that the most nearly related forms of animals do 

 present a relation of repression and advance, or of permanent em- 

 bryonic and adult type, leaving no doubt that the one is descended 

 from the other. This relation was termed exact parallelism. 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 retardation 

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

 the result of unequal acceleration or retardation; that is, accele- 

 ration affecting one organ or part more than another, thus dis- 

 turbing 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, acceleration im- 

 plies constant addition to the parts of an animal, while retardation 

 implies continual subtraction from its characters, or atrophy. He 

 had also shown (Method of Creation, 18*71), that the additions 

 either appeared as exact repetitions of preexistent parts, or as 

 modified repetitions, the former resulting in simple, the latter in 

 more complex organisms. 



Professor 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 Cor segmentation) 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 

 difference may be accounted for, on plain principles. In 18T4, 

 Haeckel, in his Anthropogenie, recognizes the importance of the 

 irregularity of time of appearance of the different characters of 

 animals, during the period of growth, as affecting their permanent 

 structure. While maintaining the view that the low forms repre- 

 sent 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 

 relation 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 



