376 THE GRAMMAR OF SCIENCE 



foolishness. Science would welcome above all things a 

 description of the action between vital units as simple as 

 the law of gravitation, provided it gave a causal account of 

 variation ; and the welcome would be none the less sincere 

 if the action showed that variation was biassed, and that 

 evolution would be irreversible, even with a reversed 

 sequence of physical environments. 



Suppose it were possible to maintain an absolutely 

 permanent physical environment for any type of life, 

 suppose further the organic environment, as far as it affects 

 this type, to remain unchanged, would or would it not 

 remain constant ? I suppose most biologists would answer 

 that the type would remain constant. But this is no 

 real reply to the bathmic evolutionist. The tendencies 

 which he insists upon may be exactly balanced by this 

 very physical and organic environment. We must put 

 the problem in another form. Have we any reason to 

 suppose that any type of life would change if the inorganic 

 and organic environments were equally favourable to each 

 and all its members ? The burden of reply now falls on 

 the bathmic evolutionist, he cannot take refuge in a mere 

 reference to a vague "inherent growth-force"; he is called 

 upon to give us quantitative evidence of the existence of 

 change in life-types without the influence of selection ; 

 just as the supporters of the selective action of environ- 

 ment are bound also to produce their numerical measure 

 of its effect. Now I think there is quantitative evidence 

 that types of life may change without the action of organic 

 or inorganic environment, i.e. solely owing to something 

 inherent in their constitution. One such factor of evolution, 

 genetic selection, I shall refer to later. Further variation 

 itself is the result of something inherent in the organism 

 and not solely in the environment,^ and those who suppose 

 evolution to be largely the result of occasional, abnormal, or 

 discontinuous variations are undoubtedly using a bathmic 



1 Variation may have a bathmic source, but stable and permanent variation 

 in a type is not a source alone of evolution. Bathmic evolutionists demand 

 variation with a continual bias, which would tend independently of selection 

 to change the type. 



