38o THE GRAMMAR OF SCIENCE 



a comparatively sudden degeneration or return to an earlier 

 stage.^ Thus tradition of acquired modifications may give 

 a progressive but a comparatively unstable change to the 

 higher types of life. It is a factor of evolution, but one 

 which requires the action of selection to become of a 

 permanent character. 



We are accordingly left with some form of selection 

 combined with inheritance as the fundamental mode of 

 describing the changes in living forms. Now this selection 

 may be of various kinds. The individual may have to 

 struggle with individuals of its own type, this is autogeneric 

 selection. Or with individuals of allied or wholly different 

 types, i.e. heterogeneric selection. Or with physical nature, 

 inorganic selection?' Or with one and all these influences 

 combined, in which case they are grouped together as 

 natural selection. One individual is better able to survive 

 or to leave more numerous and stronger progeny than 

 another under a given organic and inorganic environment, 

 it is thus said to be naturally selected, and natural selection 

 combined with heredity is Darwin's theory of evolution. 



Organic evolution is the progressive change of living 

 forms, usually associated with development of complex 

 forms from some one or more simple forms. Any cause 

 of progressive change in living forms is a factor of evolu- 

 tion. But before we can accept it as a factor we must 

 not only have shown its plausibility, but if possible have 

 demonstrated its quantitative validity. Under natural 

 selection a great variety of factors are included, and each 

 of them requires careful and independent consideration. 

 No physicist expects in the present state of science to 



1 I think the relatively quick development of the Greek and Roman civilisa- 

 tions is to be largely attributed to the tradition of acquired modifications. 

 With an alteration of environment the tradition was not maintained and those 

 civilisations collapsed. The individual Greek of Pericles' date or the Roman 

 of the Augustan age were widely different types from those of surrounding races. 

 They were products of tradition and not of inheritance, and they disappeared 

 with the loss of tradition rather than by ruthless extermination. 



- These distinctions, and even finer divisions, are far from idle. We 

 have seen that even such authorities as Huxley and Haeckel (p. 364), to say 

 nothing of Spencer, have got into confusion over human evolution by supposing 

 that natural selection, the survival of the fitter, necessarily means in the case of 

 man autogeneric selection, the struggle of an individual with his neighbours ! 



