INHERITANCE OF PARENTAL MODIFICATIONS 69 



may affect the offspring, which are but detached portions 

 of the parent. Even the effects of injuries to the parent 

 may be inherited by the offspring. Parental modifications 

 among the unicellular animals and plants must, then, often 

 be inherited. This is probably an important factor in their 

 evolution, perhaps as important as any other, though this 

 is doubtful, natural selection seeming even here to be the 

 chief factor. Among these lowly forms the whole body 

 shares in the process of reproduction. There are no special 

 parts of the body set aside for this function, while the rest of 

 the body functions as bone and muscle and gland and nerve. 

 The whole body divides, leaving no residue, so that any 

 modification in the parent may pass directly to the offspring. 



But how is it with more highly organized animals in 

 which the body is differentiated into different portions, each 

 with its special function, bone, muscle, nerve, digestive 

 organs, renal organs, and a great number more of special 

 organs and tissues ? In these higher animals, and in the 

 higher plants as well, the function of reproduction is not per- 

 formed by the body as a whole, but is given over to special 

 groups of cells, the germ cells, constituting the ovaries and 

 testes. It is these cells, and these only, which under ordinary 

 conditions give rise to new individuals. Under such circum- 

 stances the problem of the inheritance of parental modi- 

 fications is not so simple. How can the enlargement of a 

 muscle, due to exercise, so affect the germ cells, which lie 

 perhaps at a distance from the muscle in question, as to 

 cause the new individual, which shall arise from these germ 

 cells, to have the corresponding muscle in its body enlarged ? 

 The question, we see, is not a simple one. 



The germ cells in the body are the only ones which 



