BOTANY 151 



demonstrated for one-celled organisms. They may be killed, 

 but they never die. Death seems to be an acquired character, 

 a natural result of the struggle for existence among the cells 

 of a multicellular body. 



Up to this point in its life our plant has functioned only 

 for the individual for itself; every act has served to pre- 

 serve its own life, but now, by dividing, it enters upon the 

 act of reproduction. Living hitherto for itself alone, it is 

 now forced, by a mathematical necessity, to do something 

 for others, to function for the race or die. Thus in a simple, 

 homely way we find a physiological analogue shall I say 

 basis? of altruism. We see here also an illustration of the 

 world-wide truth, that the perpetuation of life is accomplished 

 only by sacrifice. 



When reproduction is complete we find two young plants 

 similar to the one with which we started. Reproduction was 

 accomplished by the separation of a portion of a mature in- 

 dividual, bearing to the separated piece the relation of parent 

 to offspring. Thus we see why the second generation is like 

 the first: it is a piece of it. We call this phenomenon 

 heredity. 



But if we look more closely we shall find that the two 

 new plants are not precisely alike; neither are they precisely 

 like the parent. This fact we call variation. And we have 

 illustrated in a simple way descent with modification. 



If we make a wider study of one-celled, green plants, we 

 shall find, on comparing them, that they vary in characteristic 

 manner. Those which resemble each other in certain definite 

 characters we place together in one group, called a kind, or 

 genus, and in this instance the generic name is Pleurococcus. 

 A more thorough study of the genus-group shows us that it 

 is in reality a complex of sub-groups, the individuals of 

 which are alike in more than generic characters. The sub- 

 groups are more intensive and less extensive than the genus. 

 We call them the special kinds, or species. The species of 



