Chap. III. MODERN SYSTEMS. 181 



I have already stated, this is an entirely unphysiological principle, inasmuch as it 

 assumes a contrast between the yolk and the embryo, within limits which do not 

 exist in nature. The Mammalia, for instance, which are placed, like all other Verte- 

 brata, in the category of the animals in which there is an opposition between the 

 embryo and the yolk, are as much formed of the whole yolk as the Echinoderms 

 or Mollusks. The yolk undergoes a complete segmentation in Mammalia, as well as 

 in Radiates or Worms, and most Mollusks; and the embryo when it makes its 

 appearance no more stands out from the yolk, than the little Starfish stands 

 out from its yolk. These simple facts, known since Sars and Bischoff published 

 their first observations, twenty years ago, is in itself sufficient to show that the 

 whole principle of classification of Vogt is radically wrong. 



Respecting the assertion, that neither Infusoria nor Rhizopoda produce any eggs, 

 I shall have more to say presently. As to the arrangement of the leading groups, 

 Vertebrata, Articulata, Cephalopoda, Mollusca, Vermes, Radiata, and Protozoa in 

 Vogt's system, it must be apparent to every zoologist conversant with the natural 

 affinities of animals, that a classification which interposes the whole series of Mollusks 

 between the types of Articulata and Worms, cannot be correct. A classification 

 based, like this, solely upon the changes which the yolk undergoes, is not likely 

 to be the natural expression of the manifold relations existing between all animals. 

 Indeed, no system can be true to nature, which is based upon the consideration 

 of a single part, or a single organ. 



After these general remarks, I have only to show more in detail, why I believe 

 that there are only four great fundamental groups m the animal kingdom, neither 

 more nor less. 



With reference to Protozoa, first, it must be acknowledged that, notwithstanding 

 the extensive investigation of modern writers upon Infusoria and Rhizopoda, the 

 true nature of these beings is still very little known. The Rhizopoda have been 

 wandering from one end of the series of Invertebrat<a to the other, without finding 

 a place generally acknowledged as expressing their true affinities. The attempt 

 to separate them from all the classes with which they have been so long associated, 

 and to place them with the Infusoria in one distinct branch, appears to me as 

 mistaken as any of the former arrangements, for I do not even consider that their 

 animal nature is yet proved beyond a doubt, though I have myself once sug- 

 gested the possibility of a definite relation between them and tlie lowest Gaste- 

 ropods. Since it has been satisfactorily ascertained that the Corallines are genuine 

 Algae, which contain more or less lime in their structure, and since there is hardly 

 any group among the lower animals and lower plants, which does not contain 

 simple locomotive individuals, as well as compound communities, eitlier free or adher- 

 ing to the soil, I do not see that the facts known at present preclude the po.ssibility 



