286 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



Since differences exist bct^Yeen the results obtained, in working out 

 the nervous system of Chiton, by different naturalists, it is thought best 



(from Gegeub. Morpli. Zeitschr. iv, April, 1877), and tlieir allies. In the first-men- 

 tioned work, the author comes to somewhat different conclusions from Brandt in regard, 

 to the details of the nervous system, both in Chiton (chiereiis) and Patella viilgata, 

 though the differences are not so fundamental as a first glance at the somewhat dia- 

 grammatic figures might suggest. In the "Anatomy" the author considers as a sepa- 

 rate phylum (AnqMneura) the Chitons, together with Ncomenla {Solenopns Sars) and 

 Chwtcderma, placing them under Vermes, while the Docoglossa and most of the Proso- 

 branchiate MoUusks form the third phylum (ArthrocochUdcs Ihr.) of the Mollusca. 

 In the later paper on Neomenia, &c., Ihering seems disi^osed to concede a more intimate 

 relation between the Fissurellidw and Limpets on the one hand and the CMtonidw on 

 the other. His figures would indicate a more near relation between Fissurella and 

 Chiton than between the latter and Patella, so far as the nervous system goes. It 

 must be borne in mind, while considering his differences with Brandt in regard to 

 Chiton, that the species examined by Ihering, Trachydermon cinereus Lowe, is one of 

 the lower forms of Chitonidce, closely related to the lowest existing genus, Lepiochiton ; 

 while that dissected by Brandt belongs to the higher of the two great groups of Poly- 

 plaviphora. It would be natural, therefore, that the nervous system of the former 

 should more nearly resemble the wormlike forms from which the Chitons may have 

 come out. and that the latter should be closer to the Limpets, which, though less 

 specialized, I can hardly doubt sprang from the same original stock. It is also within 

 the bounds of probability that in the details of the nervous system, as in all other 

 details, the characteristic variability (within certain limits) of the group of CMionida; 

 may assert itself. 



I cannot refrain from expressing, here, my conviction that there are at least two 

 points of view from which the classification of these invertebrates may be regarded in 

 a scientific sense. The army of embryologists, to whom, in these later days, wo owe 

 so much new light, with the enthusiastic self-confidence born of successful innova- 

 tions, as a general rule deny the existence of more than one scientific point of view. 

 More than one of them has dogmatically asserted that science in natural history now 

 consists in the study of embryology alone, and phylogenetic classifications deduced 

 therefrom. It has been said that careful and minute anatomical investigations and 

 histological researches based upon adult animals no longer deserve the name of sci- 

 ence. It has even been averred that the only object of classification now is the rep- 

 resentation in words of phylogenetic diagrams, or the derivative relations of animals 

 according to the particular author's hypotheses. It is therefore somewhat refreshing 

 to find that a school of naturalists is gradually forming, for whom anatomy as com- 

 pared with pure embryology has stiU some attractions. 



No one denies that a classification may be grounded exclusively upon the embry- 

 onic dovelopement, and may possess a high scientific character, nor that among the 

 higher animals such a basis must form a principal part of the foundation of any scien- 

 tific classification which may be applied to them. 



But what seems to be lost sight of by some of those who have escaped from the 

 bond.s of the Cuvierian system, is the fact that some of the derivatives from two par- 

 allel stocks may resemble one another more closely than specialized forms derived 

 from the same stock ; that in the early stages of the developement of organisms before 

 well-defined lines of specialization for the adults had been fixed by natural selection 

 and other factors, variations were necessarily rather the rule than the exception 

 among the embryonic forms, even when of common origin ; that the missing stages, 

 " abridged developement," etc., reported by most later embryologists, are, in all prob- 

 ability, the traces of the original vacillations and accelerations of primal evolution, 

 and that a truly philosophical classification must take these things into account. 



It must not be forgotten that we have to deal with results as well as methods, with 



