G. GENERAL NATURAL HISTORY AND ZOOLOGY. 277 



for, and which Darwin has to assume in order to have 

 materials from which a " natural selection " can be made. 

 Professor Cope claims that in his " Origin of Genera" it was 

 pointed out that the most nearly related forms of animals 

 present a relation of repression and advance, or a permanent 

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

 descended from the other. This relation was termed exact 

 parallelisni. 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 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 phe- 

 nomenon the term retardation was applied. It was then 

 shown that the iiiexact parallelism was the result of unequal 

 acceleration or retardation. 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 organ- 

 isms. Professor Haeckel, of Jena, has added the keystone to 

 the doctrine of evolution. Cope thinks, in his gastrma theory. 

 Prior to this generalization it had been impossible to deter- 

 mine the true relation existing between the four types of 

 embryonic growth. But Haeckel has happily determined the 

 existence of identical stages of growth (or segmentation) in 

 all of the types of eggs, the last of which is the gasti^ida, 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 1874 Haeckel, in his 

 " Anthroj^ogenie," recognized the importance of the irregu- 

 larity of time of appearance of the different characters of an- 

 imals during the period of growth, as affecting their perma- 

 nent structure. While maintaining the view that the low 

 forms represent the transitional stages of the higher, he pro- 

 ceeds to account for the want of exact correspondence exhib- 

 ited 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 characters in growth. To the orig- 

 inal, simple descent he applies the Igyyw p)ctlingenesis ; to the 

 modified and later growth, coenogenesis. The causes of the 



