haeckel's anthropogenie. 239 



organs, from the inner all tlie vegetative organs, are produced. Even 

 "when each of these primary layers splits into two secondaiy layers 

 (as is the case with all higher animals), precisely the same holds 

 good. A seeming exception to the splitting of the primary layers is 

 afforded by most vertebrates, but this is explained by the fact that 

 the recapitulation which the ontogenesis of an animal gives us of its 

 phylogenesis is not a complete one. The reasons for this have been 

 ably pointed out by Fritz Miiller. These are, firstly, that there is a 

 tendency for the record of phylogenesis to be blotted out, in conse- 

 quence of development always seeking a straighter road to the adult 

 stage; and secondly, that there is a tendency for the record to be 

 falsified by the struggle for existence which the larvse have to enter. 

 As a corollary of the first law, it is evident that the higher the 

 animal the less complete is the recapitulation. 



In consequence of this the phylogenist is compelled to adopt the 

 comparative procedure of the geologist ; and thus, by piecing together 

 phylogenetic fragments, he arrives at an approximate evolutionary 

 history of man. 



The comparison of the ontogenesis of amphioxus and the ascidia is 

 most important, as both stand on the boundary line between the 

 invertebrate and vertebrate animals. The true position of ampliioxus, 

 when first discovered, was not recognized, Pallas (1778) referring it 

 to the genus limax. Miiller was the first to give a systematic 

 description of its anatomy (1839). He assigned to it the lowest 

 place in the class pisces, pointing out, however, that it differed from 

 other fishes more than the fishes do from the amphibia. Haeckel 

 placed it in a group apart from all other vertebrata (acrania), assert- 

 ing that its structure is more different from fishes than that of the 

 fishes is from man. From its isolation we may conclude that it is 

 the last living representative of a previously existing group, although 

 the softness of its body is such as to preclude the possibility of fossil 

 remains. 



In structure amphioxus is a highly generalised vertebrate, but it 

 presents many peculiarities worthy of remark, e.g., the branchial 

 chamber, the ciliated hypobranchial furrow (thyroid gland), the ar- 

 rangement of the vascular system, of the sexual organs, and the 

 primordial renal duct. A comparison of the adult amphioxus with 

 the larval petromyzon (ammoccetes) is also very instructive. Goodsir 

 was the first to point out any relation between the ascidia and amphi- 



