(12/11. See Professor Hildebrand's excellent treatise 'Verbreitungsmittel der Pflanzen' 1873.)
From the foregoing several considerations we may, I think, conclude that in the above case of the Digitalis, and even in that of plants which have grown for thousands of generations in the same district, as must often have occurred with species having a much restricted range, we are apt to over-estimate the degree to which the individuals have been subjected to absolutely the same conditions. There is at least no difficulty in believing that such plants have been subjected to sufficiently distinct conditions to differentiate their sexual elements; for we know that a plant propagated for some generations in another garden in the same district serves as a fresh stock and has high fertilising powers. The curious cases of plants which can fertilise and be fertilised by any other individual of the same species, but are altogether sterile with their own pollen, become intelligible, if the view here propounded is correct, namely, that the individuals of the same species growing in a state of nature near together, have not really been subjected during several previous generations to quite the same conditions.
Some naturalists assume that there is an innate tendency in all beings to vary and to advance in organisation, independently of external agencies; and they would, I presume, thus explain the slight differences which distinguish all the individuals of the same species both in external characters and in constitution, as well as the greater differences in both respects between nearly allied varieties. No two individuals can be found quite alike; thus if we sow a number of seeds from the same capsule under as nearly as possible the same conditions, they germinate at different rates and grow more or less vigorously. They resist cold and other unfavourable conditions differently. They would in all probability, as we know to be the case with animals of the same species, be somewhat differently acted on by the same poison, or by the same disease. They have different powers of transmitting their characters to their offspring; and many analogous facts could be given. (12/12. Vilmorin as quoted by Verlot 'Des Varieties' pages 32, 38, 39.) Now, if it were true that plants growing near together in a state of nature had been subjected during many previous generations to absolutely the same conditions, such differences as those just specified would be quite inexplicable; but they are to a certain extent intelligible in accordance with the views just advanced.
As most of the plants on which I experimented were grown in my garden or in pots under glass, a few words must be added on the conditions to which they were exposed, as well as on the effects of cultivation. When a species is first brought under culture, it may or may not be subjected to a change of climate, but it is always grown in ground broken up, and more or less manured; it is also saved from competition with other plants. The paramount importance of this latter circumstance is proved by the multitude of species which flourish and multiply in a garden, but cannot exist unless they are protected from other plants. When thus saved from competition they are able to get whatever they require from the soil, probably often in excess; and they are thus subjected to a great change of conditions. It is probably in chief part owing to this cause that all plants with rare exceptions vary after being cultivated for some generations. The individuals which have already begun to vary will intercross one with another by the aid of insects; and this accounts for the extreme diversity of character which many of our long cultivated plants exhibit. But it should be observed that the result will be largely determined by the degree of their variability and by the frequency of the intercrosses; for if a plant varies very little, like most species in a state of nature, frequent intercrosses tend to give uniformity of character to it.
I have attempted to show that with plants growing naturally in the same district, except in the unusual case of each individual being surrounded by exactly the same proportional numbers of other species having certain powers of absorption, each will be subjected to slightly different conditions.