PROTOZOA 319 



SO on. The advance of this division of labour was more 

 and more favoured, and at length the organism was so 

 much differentiated that when it divided it gave rise, 

 not to two homogeneous halves, which could at once 

 become complete organisms in turn, but to two different 

 halves, each of which, in order to form the parts 

 wanting in it, and found in the other half, needed a 

 sort of depot, in which were found the biogens that 

 could create the half that was wanting. This store is 

 the nucleus of the cell ; in this are the basic particles of 

 each section, and thus we come to the stage of cellular 

 animals. 



As from this stage the missing parts were formed by 

 the basic particles at cleavage, which were contained in 

 the halved nucleus, the Lamarckian principle ceased to 

 act. However much the protoplasm of a protozoon is 

 modified, the daughter-cells do not receive any part of 

 the modified substance at cleavage ; it is the basic 

 particles that build up the protoplastic parts as it grows 

 into the mature animal. And if the protoplastic parts 

 are formed afresh at each segmentation by the basic 

 particles, they can only change when there is a change of 

 the letter. But we have seen that the basic particles 

 change of themselves, and are not modified by bodily 

 stimuli. 



The basic particles consist of living matter, and so 

 can change, grow and divide. In the earliest nucleated 

 organisms there can only have been a few of them, and 

 when changes occurred in them by chance, they created 

 different organisms. Let us take the case of a simple 

 protozoon with a cell-body containing four different 



