Darwin states that we can best understand “the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independency of the change of climate itself, would seriously affect the others.... In such cases, slight modifications, which in any way favored the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would have free scope for the work of improvement.” The first half of the first of these two quotations seems so plausible, that without further thought we may be tempted to give a ready assent to the second, yet the whole issue is contained in this statement. In the abstract, it undoubtedly appears true that any slightly useful modification might tend to be preserved. Whether it will, in reality, be preserved must depend on many things that should be taken into account. This question will come up later for further consideration; but it should be pointed out here, that, even assuming that one or more individuals happen to possess a favorable variation, it by no means follows that natural selection would have free scope for the work of improvement, because the question of the inheritance of this variation, and of its accumulation and building up through successive generations, must be determined before we can be expected to give assent to this argument, that appears so attractive when stated in an abstract and vague way.