PROTOZOA 305 



them to the life of the particular species. Their admir- 

 able adaptations are easily understood. They are the 

 maintainers of the species ; in a sense they are its most 

 essential element. If we recall our account of the 

 embryonic development, it is clear that the germ-cells 

 of living animals must have existed, in a sense, since 

 the beginning of life. The germ-cells are never created 

 afresh ; they are always formed by segmentation from 

 the germ-cells of the parents. Let us, for a moment, 

 ignore the fact that there are two kinds of germ-cells, 

 and consider the process of phylogeny (or ancestral 

 development) in the case of an organism that has only 

 one kind of germ-cells. These cells contain the basic 

 particles of a new organism. They divide and sub- 

 divide. One portion of the cells again receives all the 

 basic particles, the other portion only a part of them, 

 and this grows less and less as the cleavage proceeds, 

 because the organs are gradually formed. The cells of 

 the various organs cannot give birth to an entire organ- 

 ism, because they contain only a few of the basic 

 particles ; they have the most diverse functions, and 

 afterwards die off. Not so the germ -cells. These form 

 a chain that may be infinite in length. They perish, of 

 course, if the animal they are in perishes, and they can 

 also be destroyed by poison like all living matter ; but 

 under the proper conditions they go on to build up a 

 new organism, and so on. Hence they have been com- 

 pared to a root stretching under the ground, that brings 

 forth plants above the surface under certain circum- 

 stances and at certain times. These plants grow and 

 perish, but the root remains ; it grows on and on, and 



