EVOLUTION 381 



reduce all inorganic mechanism to one formula, like the 

 law of gravitation. He describes inorganic change by a 

 great variety of formulae. In the same way no biologist 

 can hope (at any rate till we have more idea of the vital 

 units and their laws of reaction) to describe all forms of 

 life change, of organic evolution, by aid of o?ie factor. 

 All we can require of him is that he shall not introduce 

 new factors until he has tested the old, or that if he does 

 introduce new factors he shall show that they are vercs 

 causes of progressive change, i.e. he shall at least give us 

 some measure of their quantitative efficiency. He has 

 now the means, and there is no excuse for him if he 

 stands behind the physicist in the certainty that his factor 

 or formula is of real descriptive value. 



§ 5. — Types : Individual and Racial 



In order the better to understand the general 

 conceptions of evolution, we must endeavour to give 

 more definite meanings to what we understand by type, 

 variation, correlation, terms we have used so far with 

 admitted vagueness. We shall best achieve our end by 

 taking some concrete case, say a beech tree or a wild 

 poppy, and we must confine our attention at first to some 

 very simple character. 



We are accustomed to distinguish one man from 

 another, one dog from another, one tree from another, 

 not only because they may occupy different parts of 

 space at the same instant (see pp. 71 and 168), 

 but because they possess what we term individuality. 

 We know that the shepherd distinguishes one sheep from 

 another, and if we consider them closely we shall find one 

 poppy plant in a corn-field differs from its neighbours. 

 But we want to give this difference a quantitative precision. 

 If we take a leaf from a beech tree we find upon it a 

 certain system of veining, a backbone with a number of 

 ribs running from it. If the leaf be taken in autumn 

 after it has ceased to grow, we can count the ribs on 



