Recent 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,[EH] "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,[EI] "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[EJ] 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, predominant activity and the consequent disruptive changes, with their abundant by-products utilized in luxuriant outgrowths 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 prepared 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 characters, though it may have been to some extent a co-operating factor. This, however, is a question apart from that of origin. Superabundant vigour may well, I think, have been a source of _origin_, not only of secondary sexual characters, but of many other forms of variation.