Evolution 



of sexual reproduction, by continually bringing about a mixture 

 of the nuclear substance, ensures the regular production of a 

 variety of types. Of these various combinations of characters 

 the few that are suited to the prevailing conditions will, for the 

 time being, constitute the dominant types. When conditions 

 change, fresh types will be available to replace them. The pro- 

 cess of conjugation is illustrated in Fig. 25. 



There are many groups of one-celled animals other than those 



FIG. 25. Stages in conjugation of Paramcecium. 



meg., The meganucleus ; mi'c., the micronucleus, which divides, and half 

 of which is exchanged ; p.b., Polar bodies, which the micronucleus 

 throws off, and which disappear. 



From Dendy's Outlines of Evolutionary Biology (Constable). 



typified in the Amoeba and the Paramcecium, but they do not 

 appear to have any significance so far as the descent of the higher 

 animals is concerned, and they therefore do not immediately 

 concern us. 



We have already mentioned that water is the life medium of 

 the slipper animalcule. It was destined to remain the natural 

 element, both of animals and of plants, throughout many subse- 

 quent stages of progress. The reason of this is not far to seek. 

 Active protoplasm consists to the extent of about three-fourths 



34 



