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 w^ith 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. VVe 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 



