PAETICULATE THEORY OF HEREDITY 243 



Our present conception of the relation of the germ- 

 plasm to developmental phenomena has then only a most 

 superficial resemblance to the older theories. The newer 

 point of view may be summed up in a few words, and has 

 in fact been stated already. First, that each gene may 

 have manifold effects on the organism, and second, that 

 every part of the body, and even each particular character, 

 is the product of many genes. The evidence for these two 

 conclusions has been so repeatedly referred to in the pre- 

 ceding pages that it is not necessary to go over it again, 

 but it may be worth while to emphasize that these two 

 conclusions are not pure speculations, but derived from 

 the evidence itself. It may also be well to point out that 

 even if the whole germ-plasm — the sum of all the genes — 

 acts in the formation of every detail of the body, still 

 the evidence from heredity shows that this same material 

 becomes segregated into two parts during the maturation 

 of the egg and sperm, and that at this time individual 

 elements separate from each other largely independently 

 of the separation of other pairs of elements. It is in this 

 sense, and in this sense only, that we are justified in speak- 

 ing of the particulate composition of the germ-plasm and 

 of particulate inheritance. 



There is a further idea deducible from well-known 

 facts of physiology that may at first sight seem to give 

 an impression that the organism is a ^ ' whole. '^ This 

 is the action of one part of the body on other parts by 

 means of substances set free in the blood, called hor- 

 mones. Many of them arise through the action of certain 

 so-called endocrine glands. But the relation here is so 

 obviously different from the problem dealt with as par- 

 ticulate inheritance that it calls for little more than 

 passing notice. It may, however, not be without interest 

 to refer to one case of the kind in which an endocrine 

 secretion depends on a genetic factor inherited in the 

 same way as are other genetic factors. There is a race 

 of poultry known as Sebrights (Fig. 107, a) in which the 



