Charles Darwin: His Life Told in an Autobiographical Chapter, and in a Selected Series of His Published Letters

by Charles Darwin

The following extracts from a note-book show that this point occurred to my father as early as 1837:

"Do not plants which have male and female organs together [i.e. in the same flower] yet receive influence from other plants? Does not Lyell give some argument about varieties being difficult to keep [true] on account of pollen from other plants? Because this may be applied to show all plants do receive intermixture."

Sprengel,[273] indeed, understood that the hermaphrodite structure of flowers by no means necessarily leads to self-fertilisation. But although he discovered that in many cases pollen is of necessity carried to the stigma of another flower, he did not understand that in the advantage gained by the intercrossing of distinct plants lies the key to the whole question. Hermann Müller[274] has well remarked that this "omission was for several generations fatal to Sprengel's work.... For both at the time and subsequently, botanists felt above all the weakness of his theory, and they set aside, along with his defective ideas, the rich store of his patient and acute observations and his comprehensive and accurate interpretations." It remained for my father to convince the world that the meaning hidden in the structure of flowers was to be found by seeking light in the same direction in which Sprengel, seventy years before, had laboured. Robert Brown was the connecting link between them, for it was at his recommendation that my father in 1841 read Sprengel's now celebrated Secret of Nature Displayed.[275]

The book impressed him as being "full of truth," although "with some little nonsense." It not only encouraged him in kindred speculation, but guided him in his work, for in 1844 he speaks of verifying Sprengel's observations. It may be doubted whether Robert Brown ever planted a more fruitful seed than in putting such a book into such hands.

A passage in the Autobiography (p. 44) shows how it was that my father was attracted to the subject of fertilisation: "During the summer of 1839, and I believe during the previous summer, I was led to attend to the cross-fertilisation of flowers by the aid of insects, from having come to the conclusion in my speculations on the origin of species, that crossing played an important part in keeping specific forms constant."

The original connection between the study of flowers and the problem of evolution is curious, and could hardly have been predicted. Moreover, it was not a permanent bond. My father proved by a long series of laborious experiments, that when a plant is fertilised and sets seeds under the influence of pollen from a distinct individual, the offspring so produced are superior in vigour to the offspring of self-fertilisation, i.e. of the union of the male and female elements of a single plant. When this fact was established, it was possible to understand the raison d'être of the machinery which insures cross-fertilisation in so many flowers; and to understand how natural selection can act on, and mould, the floral structure.

Asa Gray has well remarked with regard to this central idea (Nature, June 4, 1874):—"The aphorism, 'Nature abhors a vacuum,' is a characteristic specimen of the science of the middle ages. The aphorism, 'Nature abhors close fertilisation,' and the demonstration of the principle, belong to our age and to Mr. Darwin. To have originated this, and also the principle of Natural Selection ... and to have applied these principles to the system of nature, in such a manner as to make, within a dozen years, a deeper impression upon natural history than has been made since Linnæus, is ample title for one man's fame."

The flowers of the Papilionaceæ[276] attracted his attention early, and were the subject of his first paper on fertilisation.[277] The following extract from an undated letter to Asa Gray seems to have been written before the publication of this paper, probably in 1856 or 1857:—

" ... What you say on Papilionaceous flowers is very true; and I have no facts to show that varieties are crossed; but yet (and the same remark is applicable in a beautiful way to Fumaria and Dielytra, as I noticed many years ago), I must believe that the flowers are constructed partly in direct relation to the visits of insects; and how insects can avoid bringing pollen from other individuals I cannot understand. It is really pretty to watch the action of a humble-bee on the scarlet kidney bean, and in this genus (and in Lathyrus grandiflorus)[278] the honey is so placed that the bee invariably alights on that one side of the flower towards which the spiral pistil is protruded (bringing out with it pollen), and by the depression of the wing-petal is forced against the bee's side all dusted with pollen. In the broom the pistil is rubbed on the centre of the back of the bee. I suspect there is something to be made out about the Leguminosæ, which will bring the case within our theory; though I have failed to do so. Our theory will explain why in the vegetable ... kingdom the act of fertilisation even in hermaphrodites usually takes place sub jove, though thus exposed to great injury from damp and rain."