90 Miscellaneous. 



capacity of forminj^ a body-cavity, and yet its descendants have iu 

 some unaccountable manner (entirely against the rules of llackel's 

 theory") managed to get one by some unexplained method. We do 

 not see how it can be so confidently stated by Hiickel that Echi- 

 noderms have lost their original central nervous organ ; there is no 

 proof whatever of its once having existed. There is as yet no proof 

 whatever that the organs of sense (which, as had already been so 

 often insisted upon by Agassiz, are not homologous in the dill'erent 

 branches of the animal kingdom) have the same phylogenetic origin. 

 When Hiickel says that the mouth of Echinoderms is not homolo- 

 gous to the primitive mouth, we can only refer him to the memoirs 

 of Miiller, Metschuikoff, and myself on Echiuoderm embryos for proof 

 to the contrar)-. 



There seems no doubt, as Hiickel insists, that to the majority of 

 zoologists of the present day the idea of type is a very dift'erent 

 one from that of type as understood by Baer and Cuvier. The 

 probability of their original community of origin is hinted at from 

 the many so-called intermediate forms, both living and fossil, which, 

 though we may enroll them either in one great branch of the animal 

 kingdom or another, yet show that we can no longer consider the 

 great types of the animal kingdom as closed cycles, but must here- 

 after regard them as holding to one another relations similar to those 

 which the remaining categories of our systems have to one another. 

 This change has principally been brought about by a better know- 

 ledge of the embiTology of a few well-known types. 



Eut what becomes of all the assumptions of Hackel which form 

 the basis of his Gastroja theory ? They are totally unsupported ; 

 and with their refutation must fall his theory ; it can only take its 

 place by the side of other physiophilosophical systems ; they are 

 ingenious arrangements laboriously built up in the interests of special 

 theories, which fall to the ground the moment we test them by our 

 actual knowledge. That the time has not yet come for embryolo- 

 gical classifications, the attempts of Hiickel plainly show ; for they 

 are in no wise in advance of the other embryologieal classifications 

 which have preceded them : we get new names for somewhat differ- 

 ent combinations ; but a truly scientific basis for a classification based 

 upon the value of embryonic layers is at present impossible ; such 

 attempts can be only speculations, to be proved or disproved on the 

 morrow. 



What Hiickel substitutes in the place of the accepted types of 

 the animal kingdom is simply another view of these same types ; 

 and his Gastrcra theory is in no danger of upsetting, at present at 

 least, zoological classification as now understood. Indeed, if we 

 need an ancestor for our phylum, why not at once go back to the 

 cell? There we have a definite starting-point, a typical element 

 which underlies the whole of the animal kingdom, and which forms 

 the walls of Hiickel's Gastrula. Then we shall all bo agreed ; and 

 when we frankly state that all organisms are derived from a pri- 

 mitive cell and from its subsequent increase, wc come within the 

 range of positive knowledge, but we are unfortunately as far as 

 ever from having for that reason been able to trace a mechanical 



