352 GENERAL ZOOLOGY 



According to him the former are not inherited but the latter 

 may lead to marked changes in succeeding generations. 

 Weismann's Germ Plasm theory lays the whole responsi- 

 bility for heredity on the germ cells; they are the " vehicle 

 of inheritance" (Fig. 116). The body (soma) of each 

 individual, though carrying the germ cells, is to be looked 

 upon, from this point of view, only as a by-product of them. 

 Each generation of germ cells produces enough soma 

 (which includes all muscle, nerve, gland even the mind 

 and consciousness) to enable it to survive in the struggle 

 for existence. But the germ cells control every detail of 

 the body, and must be modified before there can be heredi- 

 tary changes in the species as a whole. Nothing in the 



-Body 





FIG. 116. Diagram illustrating germinal continuity. Through a series of 

 divisions a germ-cell gives rise to a body, or soma, and to new germ cells. The 

 latter, not the body, give rise to the next generation. (From Guyer's Being 

 W ell-Born, Copyright, 1916. By special permission of the publishers, The 

 Bobbs-Merrill Company.) 



body can be inherited. All somatic qualities are caused 

 by some modification of the germ cells, which reproduce 

 all the " hereditary" qualities by which particular species 

 are recognized in each generation. The soma dies at 

 the end of each generation but there is no break in " con- 

 tinuity of the germ plasm." 



Blending Inheritance. Animals produced from a zygote 

 formed by the union of two germ cells partake equally 

 from each parent in many of their characteristics. A large 

 rabbit, for example, when bred with a small one usually 

 produces offspring of intermediate size. In many such 

 cases of supposed blending, however, the succeeding gener- 

 ations do not retain the intermediate or blending condition 

 but are separable into several groups. The increasing 

 number of such " exceptions" has led some investigators to 



