THE CAUSATION OF VARIATIONS 99 



variations are caused by the direct action of the environment. As 

 a fact it is impossible to solve the problem experimentally. 

 An experiment cannot be made unless the animal or plant 

 used for the purpose is removed from its normal environment to 

 one very different ; and the question is, not whether it is pos- 

 sible to devise conditions in which the germ-plasm, though not de- 

 stroyed outright, is injured beyond recovery, but whether, under 

 the conditions in which species maintain their characters or undergo 

 adaptive change, variations commonly arise through the direct 

 action of the environment. In other words, the question is whether 

 variations are normally spontaneous or due to enforced alteration. 

 In the search for experimental data and the * obvious inferences ' 

 so often drawn from them, the whole problem of adaptation has 

 been actually, though not ostensibly, ignored. Men have failed to 

 make the obligatory deductive inference of consequences followed 

 by an appeal to reality. They have not asked themselves how the 

 multitudinous and delicate adaptations and co-adaptations of 

 animals and plants could have arisen if species had drifted at the 

 mercy of the environment, nor troubled their heads with such 

 notorious facts as that human beings persist and for ages have 

 persisted in lands where every individual is saturated with the 

 poisons of malaria or other virulent diseases. That is to say they 

 have not taken the whole of the facts into consideration. If the 

 reader's thinking, also, is not to be mere guessing, he must follow 

 the procedure which has led to the creation of all science that is 

 more than a mere catalogue of likenesses and differences, 1 and, bear- 

 ing in mind that species become adapted to their surroundings, ask 

 himself to what the adaptation is due, to a vital reaction resulting 

 from the Natural Selection of favourable spontaneous variations, 

 or to a helpless drift resulting from germinal changes caused by 

 the direct action of the environment. In other words, he must ask 

 himself which of these contradictory hypotheses can be conceived 

 as being in accord with the facts of existence. 



161. Some biologists have argued that influences (e.g. abundant 

 nutriment) from the environment acting on the germ-plasm tend 

 to cause variations all round the specific mean. But if the germ- 

 plasm contained in the germ-cells of the species is everywhere 

 very similar as it must be, since the individuals of the species 

 resemble one another then a force acting on it, if it causes any 

 change, must cause everywhere much the same change and there- 

 fore a general drift in this or that direction. It is only when 



