132 THE KINGDOM OF MAN 



whilst natural selection may be favouring some small 

 and obscure change in an unseen group of cells such 

 as digestive, pigmentary or nervous cells, and that change 

 a change of selective value there may be, indeed often 

 is, as we know, a correlated or accompanying change 

 in a physiologically related part of far greater magnitude 

 and prominence to the eye of the human onlooker. This 

 accompanying or correlated character has no selective 

 value, is not an adaptation is, in fact, a necessary but 

 useless by-product. A list of a few cases of this kind 

 was given by Darwin, but it is most desirable that more 

 should be established. For they enable us to understand 

 how it is that specific characters, those seen and noted 

 on the surface by systematists, are not in most cases 

 adaptations of selective value. They also open a wide 

 vista of incipient and useless developments which may 

 suddenly, in their turn, be seized upon by ever-watchful 

 natural selection and raised to a high pitch of growth 

 and function. 



The second, somewhat but by no means altogether 

 neglected, principle is that a good deal of the important 

 variation in both plants and animals is not the variation 

 of a minute part or confined to one organ, but has 

 really an inner physiological basis, and may be a varia- 

 tion of a whole organic system or of a whole tissue 

 expressing itself at several points and in several shapes. 

 In fact, we should perhaps more generally conceive of 

 variation as not so much the accomplishment and pre- 

 sentation of one little mark or difference in weight, 

 length, or colour, as the expression of a tendency to vary 

 in a given tissue or organ in a particular way. Thus 

 we are prepared for the rapid extension and dominance 

 of the variation if once it is favoured by selective 

 breeding. It seems to me that such cases as the com- 



