Chap. III. EMBRYOLOGICAL SYSTEMS. 231 



The classification of Vogt (Zoologische Briefe, q. a., p. 180) presents several /'-r/ 

 new features, one of which is particularly objectionable. I mean the separation ol 

 the Cephalopoda from the other MoUusks, as a distinct primary division of the 

 animal kingdom. Having adopted the fundamental distinction introduced by Kol- 

 liker between the animals in which the embryo is developed from the whole yolk, 

 and those in which it arises from a distinct part of it, Vogt was no doubt led 

 to this step in consequence of his interesting investigations upon Actason, in which 

 he found a relation of the embryo to the yolk differing greatly from that observed 

 by Kolliker in Cephalopods. But as I have already shown above, this cannot 

 any more justify their separation, as branches, than the total segmentation of the 

 yolk of Mammalia could justify the separation of the latter from the other Verte- 

 brates. Had the distinction made by Vogt, between Cephalopods and the other 

 Mollusks, the value he assigns to it, Limax should also be separated from the 

 other Gasteropods. The assertion that Protozoa produce no eggs, deserves no special 

 consideration after what has already been said in the preceding sections respecting 

 the animals themselves. As to the transfer of the Cteuophora to the type of 

 Mollusks, it can in no way be maintained. 



Before closing this sketch of the systems of Zoology, I cannot forego the 

 opportunity of adding one general remark. If we remember how completely inde- 

 pendent the investigations of K. E. von Baer were from those of Cuvier, how 

 different the point of view was from which they treated their subject, the one 

 considering chiefly the mode of development of animals, while the other looked 

 mainly to their structure ; if we further consider how closely the general results 

 at which they have arrived agree throughout, it is impossible not to be deeply 

 impressed with confidence in the opinion they both advocate, that the animal king- 

 dom exhibits four primary divisions, the representatives of which are organized 

 upon four difTerent plans of structure, and grow up according to four different 

 modes of development. This confidence is further increased when we perceive 

 that the new primary groups which have been proposed since are neither char- 

 acterized by such different plans, nor developed according to such different modes of 

 development, but exhibit simply minor differences. It is, indeed, a very unfortu- 

 nate tendency, which prevails now almost universally among naturalists, Avith refer- 

 ence to all Ivinds of groups, of whatever value they may be, from the branches 

 down to the species, to separate at once from one another any types which exhibit 

 marked differences, without even inquiring first whether these differences are of 

 a kind that justifies such separations. In our systems, the quantitative element 

 of differentiation prevails too exclusively over the qualitative. If such distinc- 

 tions are introduced under well-sounding names, they are almost certain to be 

 adopted ; as if science gained any thing by concealing a difficulty under a Latin 



