Ealing: October 19, 1893.
Dear Mr. Romanes,—If you are in town on November 16, I should be very glad indeed if you could come to the Linnean Society, and criticise my paper which I am going to read: 'On the origin of plant structures by self-adaptation to the environment, exemplified by desert and xerophyllous plants.'
In this and in subsequent letters Mr. Henslow explained the subject-matter of his paper, and as it formed the basis of the correspondence, a brief analysis, furnished by Mr. Henslow in a later letter, is here inserted.
The object of the paper is to show that the origin of varieties and species—as far as the vegetative organs are concerned—is solely due to climatic causes. For the acquired (somatic) characters become more or less hereditary if the same environment be maintained. But plants possess every degree in their capacities either of reverting, changing, or of stability.
The result is that I do not see any necessity for natural selection at all in Nature, for the following reasons.
Variations are often indefinite in cultivation, especially after several years. Therefore to secure a useful race artificial selection is necessary. On the other hand, variation is definite in Nature, all the seedlings varying in one and the same direction, i.e. towards equilibrium with the environmental forces. Darwin knew of this fact, and you have abundantly described it. But Darwin failed to see that this definite variation in Nature is the rule, and not the exception. Hence, as he admits, natural selection is not wanted at all [i.e. if all variations are definite in Nature].
Moreover, it is contended that climatic variations are of no great, even of any useful importance. This may be so, for all I know, with animals; but it is precisely the reverse with plants. I took my illustrations from desert plants, and showed that their remarkable characteristics, which give the facies to desert plants, are on the one hand the direct results of the excessive drought, heat, light, &c. On the other, they are just those features which enable the plants to live under their extremely inhospitable environment. These characters are the minute leaves, hardening of woody tissues, thick cuticle, dense clothing of hair, wax, storage of water tissues, &c.; so that the whole economy of the plant, including its specific characters, is all climatically acquired. Although some may vary when the plants are grown in ordinary gardens, such is no more than one would expect on a priori grounds to be the case.
I would limit natural selection, as far as plants are concerned, to three things:
1. Mortality among seedlings with the survival of the strongest.