122 HOW ANIMALS DEVELOP 



The Activity of Genes in Development 



Finally, now that we have got an idea of how devel- 

 opment comes about, we must ^o back to the question 

 which was raised in Chapter 2. How do the hereditary 

 factors or genes affect development? The first point to 

 emphasise is the enormous importance of their effects. 

 No animal can develop any characteristic for which it 

 has not got the hereditary potentialities. The nature 

 of its genes, which it inherited from its mother and 

 father, determine all the possible things it may develop 

 into. Which of these potentialities will be actually 

 realised as development proceeds depends, of course, 

 to some extent on the external circumstances or en- 

 vironment, but in general the influence of the environ- 

 ment only becomes important in affecting characters 

 which are developed relatively late in the individual's 

 lifetime. Human personality and ability are, for in-r 

 stance, strongly influenced by environment in upbring- 

 ing, because these are characters which are moulded 

 at a comparatively late stage, after birth, but whether 

 a man develops five fingers or four is very little affected 

 by the environment. It sometimes occurs, for reasons 

 of grave illness of the mother or other mishaps, that 

 conditions in the maternal uterus are so unfavourable 

 that a potentially normal child develops with some 

 anatomical abnormality of this kind, but this is a 

 relatively rare occurrence. In general, the characters 

 which are produced in the early stages of development 

 are almost entirely determined by the genes, with only 



