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, indeed, understood that the hermaphrodite structure of flowers by no means necessarily leads to selffertilisation. 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† 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.‡

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

* Christian Conrad Sprengel, born 1750, died 1816. † Fertilisation of Flowers (Eng. Trans.) 1883, p. 3.

† Das entdeckte Geheimniss der Natur im Baue und in der Befruchtung der Blumen. Berlin, 1793.

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 * attracted his attention early, and were the subject of his first paper on fertilisation.† 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) ‡ 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 order to which the pea and bean belong.

† Gardeners' Chronicle, 1857, p. 725. It appears that this paper was a piece of "over-time” work. He wrote to a friend, "that confounded Leguminous paper was done in the afternoon, and the consequence was I had to go to Moor Park for a week."

‡ The sweet pea and everlasting pea belong to the genus Lathyrus.

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 Leguminose, 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."

....

A letter to Dr. Asa Gray (September 5th, 1857) gives the substance of the paper in the Gardeners' Chronicle :

"Lately I was led to examine buds of kidney bean with the pollen shed; but I was led to believe that the pollen could hardly get on the stigma by wind or otherwise, except by bees visiting [the flower] and moving the wing petals: hence I included a small bunch of flowers in two bottles in every way treated the same: the flowers in one I daily just momentarily moved, as if by a bee; these set three fine pods, the other not one. Of course this little experiment must be tried again, and this year in England it is too late, as the flowers seem now seldom to set. If bees are necessary to this flower's selffertilisation, bees must almost cross them, as their dusted rightside of head and right legs constantly touch the stigma.

"I have, also, lately been reobserving daily Lobelia fulgens -this in my garden is never visited by insects, and never sets seeds, without pollen be put on the stigma (whereas the small blue Lobelia is visited by bees and does set seed); I mention this because there are such beautiful contrivances to prevent the stigma ever getting its own pollen; which seems only explicable on the doctrine of the advantage of crosses."

The paper was supplemented by a second in 1858.* The chief object of these publications seems to have been to obtain information as to the possibility of growing varieties of Leguminous plants near each other, and yet keeping them true. It is curious that the Papilionaceæ should not only have been the first flowers which attracted his attention by their obvious adaptation to the visits of insects, but should also have constituted one of his sorest puzzles. The common pea and the sweet pea gave him much difficulty, because, although they are as obviously fitted for insect-visits as the rest of the order, yet their varieties keep true. The fact is that neither of these plants being indigenous, they are not perfectly adapted for fertilisation by British insects. He could not, at this stage of his observations, know that the co-ordination between a

* Gardeners' Chronicle, 1858, p. 828.

flower and the particular insect which fertilises it may be as delicate as that between a lock and its key, so that this explanation was not likely to occur to him.

Besides observing the Leguminosæ, he had already begun, as shown in the foregoing extracts, to attend to the structure of other flowers in relation to insects. At the beginning of 1860 he worked at Leschenaultia,* which at first puzzled him, but was ultimately made out. A passage in a letter chiefly relating to Leschenaultia seems to show that it was only in the spring of 1860 that he began widely to apply his knowledge to the relation of insects to other flowers. This is somewhat surprising, when we remember that he had read Sprengel many years before. He wrote (May 14) :

"I should look at this curious contrivance as specially related to visits of insects; as I begin to think is almost universally the case."

Even in July 1862 he wrote to Asa Gray:

"There is no end to the adaptations. Ought not these cases to make one very cautious when one doubts about the use of all parts? I fully believe that the structure of all irregular flowers is governed in relation to insects. Insects are the Lords of the floral (to quote the witty Athenæum) world."

This idea has been worked out by H. Müller, who has written on insects in the character of flower-brceders or flowerfanciers, showing how the habits and structure of the visitors are reflected in the forms and colours of the flowers visited.

He was probably attracted to the study of Orchids by the fact that several kinds are common near Down. The letters of 1860 show that these plants occupied a good deal of his attention; and in 1861 he gave part of the summer and all the autumn to the subject. He evidently considered himself idle for wasting time on Orchids which ought to have been given to Variation under Domestication. Thus he wrote :

"There is to me incomparably more interest in observing than in writing; but I feel quite guilty in trespassing on these subjects, and not sticking to varieties of the confounded cocks, hens and ducks. I hear that Lyell is savage at me."

It was in the summer of 1860 that he made out one of the most striking and familiar facts in the Orchid-book, namely, the manner in which the pollen masses are adapted for removal by insects. He wrote to Sir J. D. Hooker, July 12 :

"I have been examining Orchis pyramidalis, and it almost equals, perhaps even beats, your Listera case; the sticky

* He published a short paper on the manner of fertilisation of this flower, in the Gardeners' Chronicle 1871, p. 1166.

glands are congenitally united into a saddle-shaped organ, which has great power of movement, and seizes hold of a bristle (or proboscis) in an admirable manner, and then another movement takes place in the pollen masses, by which they are beautifully adapted to leave pollen on the two lateral stigmatic surfaces. I never saw anything so beautiful."

In June of the same year he wrote :

"You speak of adaptation being rarely visible, though present in plants. I have just recently been looking at the common Orchis, and I declare I think its adaptations in every part of the flower quite as beautiful and plain, or even more beautiful than in the woodpecker." *

He wrote also to Dr. Gray, June 8, 1860 :—

"Talking of adaptation, I have lately been looking at our common orchids, and I dare say the facts are as old and wellknown as the hills, but I have been so struck with admiration at the contrivances, that I have sent a notice to the Gardeners' Chronicle."

Besides attending to the fertilisation of the flowers he was already, in 1860, busy with the homologies of the parts, a subject of which he made good use in the Orchid book. He wrote to Sir Joseph Hooker (July):

"It is a real good joke my discussing homologies of Orchids with you, after examining only three or four genera; and this very fact makes me feel positive I am right! I do not quite understand some of your terms; but sometime I must get you to explain the homologies; for I am intensely interested in the subject, just as at a game of chess."

This work was valuable from a systematic point of view. In 1880 he wrote to Mr. Bentham:

"It was very kind in you to write to me about the Orchideæ, for it has pleased me to an extreme degree that I could have been of the least use to you about the nature of the parts."

The pleasure which his early observations on Orchids gave him is shown in such passages as the following from a letter to Sir J. D. Hooker (July 27, 1861) :

"You cannot conceive how the Orchids have delighted me. They came safe, but box rather smashed; cylindrical old cocoa- or snuff-canister much safer. I enclose postage. As an account of the movement, I shall allude to what I suppose is Oncidium, to make certain, is the enclosed flower with crumpled petals this genus? Also I most specially want to know what the enclosed little globular brown Orchid is. I have

* The woodpecker was one of his stock examples of adaptation.