Russel Wallace : Alfred Russell Wallace (sic)
In order to avoid needless trouble and misunderstanding of the real point at issue, I ask to be allowed to state the problem as it appears to me.
In the first place, every farmer and gardener knows that plants raised from seed vary immensely, and that it is by the selection of variations of this kind in the desired direction that all the improved varieties of our flowers and vegetables have been produced, "because such variations are more or less hereditary." These are "congenital" or "germ" characters, and of their transmission to the offspring there is no question. But many, perhaps most, of these characters only appear in the adult individual as variations of the flower and fruit, and thus variations of this kind may appear to be due to external conditions without being really so. Again, it is so constantly the practice of gardeners, &c., to select from among such variations those that suit their purpose, while in other cases the forces of Nature bring about a corresponding selection, that this is apt to be overlooked, and thus the effects of selection among spontaneous or congenital variations get mixed up with the effects of changed conditions acting on individual plants or animals, producing changes of form or structure, which changes may conceivably be either confined to these individuals or be transmitted to their offspring. Every gardener and farmer modifies the plants he grows by supplying them with special conditions which produce results favourable to his special purpose. Such are the various manures, the particular soils, the supply of water, of shelter from winds, of shade or sunshine, the time of sowing, &c.; and the question is whether changes thus produced in the individual are transmitted, as a rule, to the offspring. To determine this is not easy, because, as I have said, the cultivator always selects also, and perhaps no experiment has ever yet been made in which selection has been rigidly excluded. In order to do so, some such course as the following must be tried:--
Let seed of some easily grown plant be taken and divided fairly into two equal portions. Let each set of plants may be submitted to a diverse set of conditions during its whole life, but always so regulated as to allow both sets to grow healthily and to produce flowers and seeds. Next year let the whole of the seeds from each lot be again sown and subjected to the same conditions, or if the seed is more than required, let the same quantity of each be taken by some process which will not select the best seeds in any way, but will take a true average sample. In the second year a very considerable difference will, no doubt, be seen in the two lots grown under diverse conditions, and to determine if this difference acquired by the individuals is in any degree hereditary, a fair sample of the seed of each must now be sown side by side, under as near as possible identical conditions. If now the difference which had been produced in the stem, foliage, flowers, or fruits of the two lots grown under different conditions is maintained when the seed is grown under the same conditions--selection at any stage having been carefully avoided--then there would be some evidence that individually acquired characters may be, and are actually inherited.
With regard to Mr. Henslow's main theory that irritations set up by insects have modified structures, and that these modifications have been inherited, I would ask whether plants that have for many generations produced galls under the irritation of insects have ever been known to produce galls when the visits of the gall-producing insect have been absolutely prevented? If the results of insect irritation are in any degree inherited, we ought surely to find some plants which constantly produce growths resembling those of the special galls of the species, but without any insect agency having intervened.
ALFRED R. WALLACE.