Moreover, when dealing even with such comparatively slight changes as occur between our domesticated varieties--and which, _a fortiori_, are less likely to become "stable" through the uniform operation of causes other than selection, seeing that they are not only smaller in amount than occurs among natural species, but also have had but a comparatively short time in which to accumulate--Darwin is emphatic in his assertion of the same principles. For instance, in the twenty-third chapter of the _Variation of Plants and Animals under Domestication_, he repeatedly uses the term "definite action of external conditions," and begins the chapter by explaining his use of the term thus:-- "By the term definite action, as used in this chapter, I mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, _all_, or _nearly all_, the individuals are modified in the same manner. A new _sub-variety_ would thus be produced _without the aid of selection_[170]." [170] _Variation_, &c., vol. ii. p. 260. As an example of the special instances that he gives, I may quote the following from the same work:-- "Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on many individuals, _all_ probably would be modified in the same manner." And, as instances of his more general statements in Chapter XXIII, these may suffice:-- "The direct action of the conditions of life, whether leading to definite or indefinite results, _is a totally distinct consideration from the effects of natural selection_.... The direct and definite action of changed conditions, in contradistinction to the accumulation of indefinite variations, _seems to me so important_ that I will give a large additional body of miscellaneous facts[171]." [171] _Ibid._ vol. ii. p. 261. Then, after giving these facts, and showing how in the case of species in a state of nature it is often impossible to decide how much we are to attribute to natural selection and how much to the definite action of changed conditions, he begins his general summary of the chapter thus:--