306 THE THEORY OF THE GENE 



series of processes. A change that affects any step in the 

 process may be expected often to affect a change in the 

 end-resnlt. It is the final visible effect that we see, not 

 the point at which the effect was brought about. If, as we 

 may readily suppose, very many steps are involved in the 

 development of a single organ, and if each of these steps 

 is affected by the action of a host of genes, there can be 

 no single representative in the germ-plasm for any organ 

 of the body, however small or trivial that organ may be. 

 Suppose, for instance, to take perhaps an extreme case, 

 all the genes are instrumental in producing each organ 

 of the body. This may only mean that they all produce 

 chemical substances essential for the normal course of 

 development. If now one gene is changed so that it pro- 

 duces some substance different from that which it pro- 

 duced before, the end-result may be affected, and if the 

 change affects one organ predominatingly it may appear 

 that one gene alone has produced this effect. In a strictly 

 causal sense this is true, but the effect is produced only 

 in conjunction with all the other genes. In other words, 

 they are all still contributing, as before, to the end-result, 

 which is different in so far as one of them is different. 



In this sense, then, each gene may have a specific effect 

 on a particular organ, but this gene is by no means the 

 sole representative of that organ, and it has also equally 

 specific effects on other organs, and, in extreme cases, 

 perhaps on all the organs or characters of the body. 



To return now to our comparison. The effect of a 

 change in a gene (which if recessive means, of course, .a 

 pair of like genes) frequently produces a more localized 

 effect than a doubling or trebling of the genes already 

 present, because a change in one gene is more likely to 

 upset the established relation between all the genes than 

 is an increase in the number of genes already present. By 

 extension, this argument seems to mean that each gene 



