After pointing out that naturalists have no definite standard to determine whether a group of individuals is a variety or a species, Darwin makes the highly important admissions contained in the following paragraph: “Hence, I look at individual differences, though of small interest to the systematist, as of the highest importance for us, as being the first steps toward such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps toward more strongly marked and permanent varieties; and at the latter, as leading to subspecies, and then to species. The passage from one stage of difference to another may, in many cases, be the simple result of the nature of the organism and of the different physical conditions to which it has long been exposed; but with respect to the more important and adaptive characters, the passage from one stage of difference to another may be safely attributed to the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or disuse of parts. A well-marked variety may therefore be called an incipient species; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work.”
In this paragraph attention should be called especially, first, to the statement in respect to the origin of varieties, which are said to arise through individual differences. It is not clear whether these differences are supposed to have appeared first in one, or in a few individuals, or in large numbers at the same time. Again, especial note should be made of the striking admission, that the passage from one stage to another may, in many cases, be the simple result of the nature of the organism and of the physical conditions surrounding it; but with respect to the more important and adaptive differences, natural selection “may safely” be supposed to have intervened. Is it to be wondered at that Darwin’s critics have sometimes accused him of playing fast and loose with the origin of varieties? And since this question is fundamental for the theory of natural selection, it is much to be regretted that Darwin leaves the matter in such a hazy condition. It may be said that, at the time when he wrote, he made the best of the evidence in regard to the origin of varieties. Be this as it may, a theory standing on no better foundations than this is not likely to be found satisfactory at the present time.