8o ORGANIC EVOLUTION 



bears it, and is to a considerable degree protected from con- 

 tact with the outer world. Why, then, does the egg change 

 its constitution ? 



Those who are at all familiar with biological phenomena 

 know that all living things and all parts of their bodies are 

 constantly changing. No bit of living protoplasm is ever 

 at rest. It liberates the energy used in its different life 

 activities only by the destruction of some of its sub- 

 stance, and this constant waste has to be constantly 

 repaired. For this repair food is needed and is digested and 

 assimilated, being built up into new protoplasm to take the 

 place of that which was destroyed. Changes in nutrition 

 may cause changes in the constitution of the organism which 

 is being nourished. The constitution of the germ cells may 

 thus vary with the changing conditions of nutrition, and such 

 changes in the structure of the germ cells may register them- 

 selves in changes in the organisms which arise from these 

 germ cells. Variation in animals and plants may therefore 

 be due to the conditions of nutrition of the germ cells from 

 which they came. 



Germ cells receive their nutriment from the blood or 

 lymph in all higher animals. The blood may contain other 

 substances than food which will affect the character of the 

 germ cells. Changes in the blood other than those con- 

 nected with nutrition may therefore cause changes in the 

 germ cells, producing variation in the offspring. Such 

 changes in the constitution of the blood may be due to 

 the action of the somatic cells, since their waste products 

 and secretions find their way into the blood. One can 

 readily conceive, for example, that imperfect action of the 

 renal cells (perhaps due to disease), resulting in impure 



