Organic Evolution. 237 



nature of the flame depending on the combustible matter, 

 and not on the spark." 



Eecent investigations have certainly not lessened the 

 force of Darwin's contention. From which there follows 

 the corollary that the vital condition of the organism is a 

 fact of importance. Darwin was led to believe that among 

 domesticated animals and plants good nutritive conditions 

 were favourable to variation. " Of all the causes which 

 induce variability," he says,* " excess of food, whether or 

 not changed in nature, is probably the most powerful." 

 Darwin also held that the male is more variable than the 

 female a view that has been especially emphasized by 

 Professor W. K. Brooks. Mr. Wallace, as we have already 

 seen, regards the secondary sexual characters of male birds 

 as the direct outcome of superabundant health and vigour. 

 "There is," he says,f "in the adult male a surplus of 

 strength, vitality, and growth-power which is able to 

 expend itself in this way without injury." And Messrs. 

 Geddes and Thomson contend* that "brilliancy of colour, 

 exuberance of hair and feathers, activity of scent-glands, 

 and even the development of weapons, are in origin and 

 development outcrops of a male as opposed to a female 

 constitution." 



There is, I think, much truth in these several views 

 thus brought into apposition. Vigour and vitality, pre- 

 dominant activity and the consequent disruptive changes, 

 with their abundant by-products utilized in luxuriant out- 

 growths and brilliant colours, are probably important 

 sources of variation. They afford the material for natural 

 selection and sexual selection to deal with. These guide 

 the variations in specific directions. For I am not pre- 

 pared to press the theory of organic combination so far as 

 to believe that this alone has served to give definiteness 

 to the specific distinctions between secondary sexual charac- 

 ters, though it may have been to some extent a co-operating 

 factor. This, however, is a question apart from that of 



' * " Animals and Plants under Domestication," vol. ii. p. 244. 

 f " Darwinism," p. 293. $ " Evolution of Sex," p. 22. 



