SECTION D— ZOOLOGY. 



NATURAL SELECTION AND 

 EVOLUTIONARY PROGRESS 



ADDRESS BY 



J. S. HUXLEY, M.A., D.Sc, 



PRESIDENT OF THE SECTION. 



The Multiformity of Evolution. 



Biology at the present time is embarking upon a phase of synthesis after 

 a period in which new disciplines were taken up in turn and worked out 

 in comparative isolation. Nowhere is this movement towards unification 

 more likely to be fruitful than in the many-sided topic of evolution ; 

 and already we are seeing its firstfruits in that reanimation of Darwinism 

 which is such a striking feature of post-war biology. 



With the reorientation made possible by modern genetics, evolution 

 is seen to be a joint product of mutation and selection. Contrary to the 

 view of Darwin and the Weismann school, selection alone has been shown 

 to be incapable of extending the upper limit of variation, and therefore 

 incapable by itself of causing evolutionary change. Contrary to the 

 views of the more extreme mutationists and the believers in ortho- 

 genesis, mutation alone has been shown to be incapable of producing 

 directional change, or of overriding selective effects. The two processes 

 are complementary. 



The students of a particular aspect of evolution are prone to think 

 that their conclusions are generally applicable, whereas they usually are 

 not. The palaeontologists unearth long evolutionary series and claim that 

 evolution is always gradual and always along a straight course, which may 

 be either adaptive or non-adaptive. However, as Haldane has pointed out, 

 their conclusions apply almost entirely to abundant and mostly to marine 

 animals. In some land plants, on the contrary, we now have evidence of 

 a wholly different method of evolution — namely, the discontinuous and 

 abrupt formation of new species. And in rare forms the course of evolu- 

 tion will not run in the same way as in abundant and dominant types. 



Meanwhile the naturalist and the comparative physiologist are struck 

 by the adaptive characters of animals and plants : to them the problem of 

 evolution becomes synonymous with the problem of the origin of adapta- 

 tion, and natural selection is erected into an all-powerful and all-pervading 

 agency. The systematist, on the other hand, struck by the apparent 

 uselessness of the characters on which he distinguishes species and genera, 

 is apt to overlook other characters which are adaptive but happen to be 

 of no use in systematics, and to neglect the broad and obviously adaptive 

 characters seen in larger groups and in palaeontological trends. 



