1892.] MICKOSCOPICAL JOUliJNAL. 241 



any other hypothesis than that of genetic affinity. Why should 

 all these animals be made of protoplasm ? Why should all have 

 similar powers and structure ? Why should all have but one 

 nucleus ? Why should we find pseudopodia, flagella, or cilia, and 

 these similar and comparable structures ? If we regard them of 

 kin, then we can refer these cases to the general law of heredity. 

 No other rational explanation of the classification of animals has 

 ever been conceived. One suggestion was made by Prof. Louis 

 Agassiz in his essay on classification. It was that classification, 

 by which term I mean the sum of facts of resemblance and dis- 

 similarity in animals, was due to the fiict, assumed by him, that 

 animals were each species separately created by a being, who, like 

 an architect, had certain plans in mind and made animals on those 

 plans. But the study of classification receives no light upon any 

 such hypothesis, the fiicts do not lead toward such a conception ; 

 for that individuality so conspicuous in an architect's productions 

 is wanting, and the species shade into one another by the most 

 insensible and indefinable gradations. No more convincing proof 

 that the facts do not lead towards Professor Agassiz's theory can 

 be found than the fact that the theory lives merely as a matter of 

 history, a curiosity of speculation, while the genetic hypothesis 

 bears such a stamp of genuineness that it lias been universally 

 adopted by the scientific world. 



If the evolution hypothesis be adopted the difterences among 

 the Protozoa at once become comparatively intelligible as spe- 

 cializations of difterent protoplasmic activities. In fact the same 

 would probably be held by an adherent of the special-creation 

 hypothesis. The phylum as a whole is thus an assemblage of 

 difterent organisms, animal in nature, in which the single cell now 

 becomes subordinated in any considerable degree, if at all, to an 

 aggregate of cells. All the higher phyla above the Protozoa are 

 made up of animals which begin life, to be sure, as single cells, 

 but in which these single cells or ova must develop into large 

 colonies of interdependent cells and form an organism. In the 

 Protozoa the specializations of protoplasmic powers produce 

 variety of kind ; in the Metazoa the specializations of cell powers 

 produce variety of tissue, as we have already seen in the frog. The 

 Protozoa as a phylum, then, is marked by the lack of any tendency 

 among the cells to co-operate. In consequence large compact 

 bodies cannot be built up, a great variety of station cannot be oc- 

 cupied, and the limits of life are perforce narrow. Two main 

 lines of specialization are at once manifest in the Protozoa — the 

 high specialization of the metabolic function on the one hand and 

 the high specialization of the sensory and motor function on the 

 other are notable in the two large classes, Rhizopoda and Infu- 

 soria. In the former the production of a skeleton is the outcome 

 of the operation of the metabolic function. This power would 

 arrive at the production of a membraneous-supporting structure 

 first, because its chemical nature is less unlike that of the proto- 



