256 



THE EVOLUTION THEORY 



and then dividing many times in rapid succession, so that a, 4, 8, 

 1 6, &c. individuals arise consecutively, and subsequently burst forth 

 from the cyst (Fig. 6j, B). In the Gregarines and other Sporozoa the 

 period of division lasts much longer, and the encysted animal divides 

 into 138, 256, or even more portions; but in this case also each part 

 or ' spore ' receives a piece of the maternal cell-body and cell-nucleus, 

 so that there is no difference in principle between this and the simple 

 division into two exhibited by Stentor ; as in that ease, so here, it is 

 not the fully differentiated structure of the animal which is handed on 

 to the divided parts ; it is only the power to redevelop this anew on 

 their own account. Thus here again we are face to face with the 

 fundamental problem of heredity : How is it possible that the power 

 of reproducing the complex whole can be inherent in the simple parts ? 



ma 



Fig. 61. Holophrya muliifliis, an Infusorian parasitic on the 

 skin of fishes. A, in its usual condition ; ma, macronucleus ; mi, 

 micronucleus ; ci', contractile vacuole ; m, mouth. B, after binary 

 fission has been several times repeated within the cyst {cy) ; tt, 

 results of the division. C, one of these units much enlarged. 



In contrast to the unicellular organisms, the great majority of the 

 multicellulars, the Metazoa and Metaphyta, many-celled animals and 

 plants, differ not only in the multitude of their cells, but even more 

 in the manifold differentiation of these cells according to the principle 

 of division of labour, so that the various functions of the animal are 

 not performed by all the cells uniformly, but each function is relegated 

 to a particular set of cells specially organized with reference to it. 

 Thus there is differentiation between motile, nutritive, and reproductive 

 cells, and there may also be glandular, nerve, muscle, and skin cells, 

 and we know how this differentiation into a great number of different 

 kinds of cells with highly specialized functions has arisen, especially 

 among the higher animals, in a multiplicity which cannot easily be 



