34 PROBLEMS OF GENETICS 



mechanical phenomena by which the cells and their constituents 

 are divided and separated. When therefore we speak of the 

 essential phenomena of heredity we mean the mechanics of divi- 

 sion, especially, though not, as we shall see, exclusively, of cell- 

 division ; and in the relation between the two halves of the divid- 

 ing cell we have the problem presented in what seems to be its 

 simplest form. 



In attempting to form some conception of the processes by 

 which bodily characteristics are transmitted, or — to avoid that 

 confusing metaphor of "transmission" — how it comes about 

 that the offspring can grow to resemble its parent, continuity of 

 the germ-substance which in some animals is a visible phenom- 

 enon, 2 gives at least apparent help. An egg for example on be- 

 coming adult develops in certain parts a particular pigment. 

 The eggs of that adult when they reach the appropriate age de- 

 velop the same pigment. We have no clear picture of the 

 mechanism by which this process is effected, but when we realise 

 that the pigment results from the interaction of certain sub- 

 stances, and that since all the eggs are in reality pieces of the same 

 material, it seems, unless we inquire closely, not unnatural that 

 the several pieces of the material should exhibit the same colours 

 at the same periods of their development. The continuity of 

 the material of the germs suggests that there is a continuity 

 of the materials from which the pigment is formed, and that 

 thus an actual bit of those substances passes into each egg 

 ready at the appropriate moment to generate the pigment. 

 The argument thus outlined applies to all substantive character- 

 istics. In each case we can imagine, if we will, the appearance 

 of that characteristic as due to the contribution of its rudiment 

 from the germ tissues. 



When we consider more critically it becomes evident that 

 the aid given by this mental picture is of very doubtful reality, 

 for even if it were true that any predestined particle actually 

 corresponding with the pigment-forming materials is definitely 



2 From the recent discoveries of Erwin Baur we are led to surmise that in the 

 flowering plants the sub-epidermal layer, or some of its elements, may legitimately 

 be regarded as a similar germ-substance, continuous in Weismann's sense. 



