122 HOW ANIMALS DEVELOP 



The Part Played by the Genes in Development 



Finally, now that we have got an idea of how 

 development comes about, we must go back to the 

 question which was raised in Chapter n. How do the 

 hereditary factors or genes affect development? As 

 a matter of fact we have very little definite experi- 

 mental evidence on this matter, because as bad luck 

 will have it, the most suitable animals for embryo- 

 logical investigations, such as the amphibians, are 

 just those animals about whose heredity we know 

 least, because they breed so slowly and are so 

 difficult to rear to sexual maturity. So all we can 

 do is to make a few guesses. 



In the first place, we know rather more about the 

 effect of genes on the last, functional period of 

 development, which has been the subject of this 

 chapter, than about their effects on the earlier 

 periods. In this last period, all sorts of details are 

 being worked out, and sometimes it is not difficult 

 to find out what factors are involved. For instance, 

 if pigment is being made, in an animal's eyes or 

 hair or in a flower's petals, we know more or less 

 what chemical processes are going on. We find that 

 genes affect such processes in at least two obvious 

 ways, either by altering the quantity of some sub- 

 stance, or by altering the rate at which the reactions 

 go on. Probably both these effects are actually due 

 to the same sort of cause; different genes cause 

 different amounts of special substances to be 

 produced, and some of these substances may 



