[6] The passage proceeds to show that in view of this consideration we have a strong additional reason for rejecting the _a priori_ dogma that all specific characters must necessarily be useful characters. For it is evident that any divergence of specific character which is brought about in this way need not present any utilitarian significance--although, of course, natural selection will ensure that it shall never be deleterious. The name which I gave to this cause of specific change was Independent Variability, or variability in the absence of overwhelming intercrossing. But it now appears to me that this cause is really identical with that which was previously enunciated by Delbœuf. Again, in his important essay on _The Influence of Isolation_, Weismann concludes, on the basis of a large accumulation of facts, that the constancy of any given specific type "does not arise suddenly, but gradually, and is established by the promiscuous intercrossing of all individuals." From which, he says, it follows, that this constancy must cease so soon as the condition which maintains it ceases--i. e. so soon as intercrossing (Panmixia) between all individuals ceases, or so soon as a portion of a species is isolated from its parent stock. To this principle he assigns the name of Amixia. But Weismann's Amixia differs from my Independent Variability in several important particulars; and on this account I have designedly abstained from adopting his term. Here it is enough to remark that it answers to the generic term Isolation, without reference to the _kind_ of isolation as discriminate or indiscriminate, homogamous or apogamous. On the other hand, my Independent Variability is merely a re-statement of the so-called "Law of Delbœuf," which, in his own words, is as follows:-- One point, however, is definitely attained. It is that the proposition, which further back we designated paradoxical, is rigorously true, A constant cause of variation, however insignificant it may be, changes the uniformity [of type] little by little, and diversifies it _ad infinitum_. From the homogeneous, left to itself, only the homogeneous can proceed; but if there be a slight disturbance ["léger ferment"] in the homogeneous, the homogeneity will be invaded at a single point, differentiation will penetrate the whole, and, after a time--it may be an infinite time--the differentiation will have disintegrated it altogether.