Life of Plants and Animals 201 



implies protoplasmic waste, which, even though made 

 good by nutrition, leaves some trace. A constantly 

 shifting environment incident to the multiplication 

 of cells, therefore, naturally leads to differentiation 

 in the nature of each cell. Heterogeneity thus re- 

 sults from comparative homogeneity. Heterogeneity, 

 whether arising from cellular interaction, from re- 

 action to external influences, or from a qualitative 

 differentiation due to unequal nuclear division, leads 

 ultimately to the appearance of the specialized tissues, 

 of which all higher forms are composed. 



Heredity and Variation. 



The specific property of the original germ-cell, 

 giving it a certain specific power of reaction to its 

 conditions of existence, is the hereditary property of 

 the cell. What this hereditary property consists of, 

 whether it be due to specific chemical substances 

 or to a primitive protoplasmic structure, an organiza- 

 tion far more complex than any known chemical 

 formula, is a very important, but as yet a disputed, 

 question. 



We know that a hereditary element of some kind 

 does exist. It is due to this hereditary property, 

 whether of the nature of a complex chemical sub- 

 stance or a primitive organization, that the long 

 series of actions and reactions to intrinsic and extrinsic 

 forces finally leads to the formation of an organism 

 resembling the species from which the original germ- 

 cell is derived. 



Admitting a hereditary element, therefore, in the 

 original germ does not necessarily imply a total 

 denial of the modifying effect of external influences 

 even on the germ, since both intrinsic and extrinsic 

 factors are involved in the final result. 



VARIATION. If the extrinsic and the intrinsic 

 factors, or the hereditary element and the environ- 



