THE DIFFERENTIATION OF SPECIES. 



23 



exceptional cases that there is none), the weeding out of those individuals who, from some 

 cause or otlier, have neglected to adapt themselves to inexorable circumstances, materially 

 increases the rapidity at which evolution progresses. 



So far everything is plain sailing, but a difficulty presents itself. What is the cause 



of variation? If a Canary be fed exclusively on Cayenne pepper it becomes scarlet; if a 



Bullfinch be fed exclusively on hempseed it becomes black. It can be no advantage to the 



Canary to become red, or to the Bullfinch to become black. This is the direct action of 



food. If a wild Duck be tamed, in a fevs^ generations its wings decrease in size and its legs 



become larger. The housewife, in choosing her Ducks for the pot, does not select the 



longest-winged or the weakest-legged birds. This is the result of use and disuse. One 



great cause of variation is unquestionably the action of surrounding circiunstances ; but 



there are variations which appear to be independent of surrounding circumstances. How 



is it possible for any voluntary action on the pait of a bird to cause spots to appear on its 



eggs, or how can surrounding circumstances modify the deposit of colour from the glands 



in the oviduct of a bird ? The only possible answer to this question is, that organisms 



have the power of adapting themselves, or of being adapted by outward circumstances within 



certain limits, to altered conditions. But what appears much more remarkable is that they 



evidently have also the power of varying or of being varied in a direction which, to all 



appearances, is not of any present advantage, but which the accumulated results of several 



generations make eventually of great advantage. It is impossible to imagine that selection 



of any kind can operate in these cases ; and whether the mysterious power lie in the embryo, 



or in the individual cells or protoplasm, which collectively are the embryo — or whether the 



wishes of the mother have power to affect the development in her womb, as Sir Kenelm 



Digby and the old school of astrologers and alchemists believed — it is probably a force or 



a power as absolutely inherent in organic life as the attraction of gravitation is in inorganic 



matter. We can no more explain the one than we can the other. Facts like these have 



to be accepted as axioms. We prove their truth by observation ; we cannot prove their 



truth by the aid of our reasoning powers ; they are the premisses upon which we reason. 



The fact that offspring do vary from their parents in points which appear to be in- 

 capable of being affected by external surroundings, but so simultaneously^ that the variations 

 are not swamped by interbreeding, and therefore cannot be ascribed to accident, seems to be 

 incontrovertible. That many of these variations are of a distinctly utilitarian character is 

 equally obvious. To suppose that the wishes of the mother have power to affect the 

 development of the embryo would only be to shift the difficulty a stage backward. It 

 would be as impossible to explain the influence of the mother as the fact of the variation 

 of the embryo. Perhaps some day embryologists will be able to explain this mystery, 



Causes of 

 variation. 



Their m3'S- 

 terions cha- 

 racter. 



Ahsurd ex- 

 planations. 



' Mr. Komanes (' Nineteenth Century,' No. 119, p. 71) says that I am "suhlimely ignorant" of this fact. 

 The only alteration I have made in the text of my paper read at the British Association is to emphasize the 

 statement by means of italics. 



