PRINCIPAL SYSTEMS OF ZOOLOGY 257 



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 Vertebrates. 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 Ctenophora 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 remem- 

 ber how completely independent 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 kingdom exhibits four primary di- 

 visions, the representatives of which are organized upon four different 

 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 characterized by such different plans, nor developed accord- 

 ing to such different modes of development, but exhibit simply 

 minor differences. It is indeed a very unfortunate tendency, which 

 prevails now almost universally among naturalists with reference 

 to all kinds 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 separa- 

 tions. In our systems the quantitative element of differentiation 

 prevails too exclusively over the qualitative. If such distinctions are 

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

 adopted; as if science gained anything by concealing a difficulty 



