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indeed, is scarcely mentioned in the “ Origin,” but Mr. Darwin has dealt with this subject in a subsequent work, the “ Descent of Man." In this he has boldly grappled with the question. He points out that man is constructed on the same type or model as other mammals—the bones in his skeleton, as well as his muscles, nerves, blood-vessels, and internal viscera, following the same law. Even his brain, as shown by Huxley and other naturalists, so closely resembles that of the quadrumana, that, as Bischoff, who is a hostile witness, admits, every chief fissure and fold in the brain of man has its analogy in that of the orang, though it is no doubt true that at no period of development do their brains perfectly agree.
Mr. Darwin points out that, even on minor points, the similarities are very striking ; such, for instance, as the arrangement of the hair on the arms. In the case of the orang, this serves to throw off the rain, when, as is the custom of this animal, the arms are bent, with the hands clasped round a branch or over its own head. If the above explanation be correct, the hair on the human forearm assumes an unexpected significance, and offers a curious record of our former state, since, as Mr. Darwin observes, no one supposes that is now of any use in throwing off the rain, nor, in our present erect condition, is it properly directed for this purpose. Again, in every large collection of human skulls some may be found with the canine teeth projecting beyond the others, in the same manner as, though to a less degree than, in the anthropomorphous apes. "He," urges Mr. Darwin, “who regards with scorn the belief that the shape of his own canines, and their occasional great development in other men, are due to our early progenitors having been provided with those formidable weapons, will probably reveal by sneering the line of his descent. For though he no longer intends, nor has the power, to use these teeth as weapons, he will unconsciously retract his snarling muscle' (thus named by Sir C. Bell) so as to expose them ready for action, like a dog prepared to fight.”
The main result at which Mr. Darwin arrives is that man is descended from some more lowly form, though he warns his readers not to suppose that our early progenitors were identical with, or even closely resembled, any existing ape or monkey. Such conclusions are, no doubt, highly distasteful to many minds, but, as Mr. Darwin points out, “we are not here concerned with hopes or fears, but only with the truth as far as our reason allows us to discover it. I have given," he adds, "the evidence to the best of my ability :" and, whatever the ultimate verdict may be, no one will deny that he has treated this question with the greatest ability, and most laudable candour.
In the “Origin of Species,” Mr. Darwin derived a strong argument from the changes which had been produced by man in domesticated animals and plants. For if considerable modifications had been thus produced during a comparatively short period, it was the less improbable that still greater alterations might have been produced by natural causes, acting through the far longer periods of geological time. In the year 1868, he published a special work on the “ Variation of Animals and Plants under Domestication.” In it he gave, under the head of each species, the facts which he had been able to collect, showing the amount and nature of the changes which animals and plants had undergone whilst under the dominion of man.
At the conclusion of the work Mr. Darwin set forth his “ provisional hypothesis," as he calls it, of Pan-Genesis, namely, that “the whole organisation, in the sense of every separate atom or unit, reproduces itself. Hence ovules, or pollen grains, the fertilised seed or egg, as well as buds, include and consist of a multitude of gemmules thrown off from each separate atom of the organism." It is universally admitted that cells propagate themselves by self-division, and Mr. Darwin assumes that, besides this means of increase, they throw off minute gemms or atoms, which circulate freely throughout the system, and multiply by selfdivision, subsequently becoming developed into cells like those from which they were derived. He supposes that these gemmules are transmitted from the parents to their offspring, and are generally developed in the succeeding generation; but are sometimes transmitted in a dormant state during many generations.
At the close of the last century, Sprengel, a German naturalist, published a most suggestive work on flowers, in which he pointed out the curious relations existing between these and insects, and showed that the latter carry the pollen from flower to flower. Sprengel's observations, however, attracted little notice, until Mr. Darwin called attention to the subject.
The first of Mr. Darwin's important contributions to Botanical Science was his “Memoir on the Genus Primula," published in the Linnean Journal for 1862. It had long been known, not only to botanists, but even to village children, that the cowslip and primrose exist under two forms, about equally numerous, and differing from one another in the arrangement of their stamens and pistils; the one form having the stamens at the summit of the flower, and the stigma half-way down; while in the other the relative positions are reversed, the stigma being at the summit of the tube, and the stamens half-way down. This difference had, however, been regarded as a case of mere variability; but Mr. Darwin showed it to be a beautiful provision, by means of which insects unconsciously fertilise each flower with pollen brought from a different plant.
These two forms of Primula differ not only in the above points, but in several others, and especially in the form and size of the pollen-grains. By a series of most careful and elaborate observations and experiments,
Mr. Darwin showed that flowers fertilised with pollen from the other form yield more seed than if fertilised by pollen of the same form, even if taken from a different plant. This
paper led to more extended researches on the subject, collected in his work on “The Different Forms of Flowers on Plants of the same Species ;” for “dimorphism," as he proposed to call it, turned out to be
" by no means confined to primulas, but to exist in many
generanay, in the case of Lythrum salicaria, the facts are even more complex, there being three distinct forms. It must not, however, be supposed that in all cases where a species has two distinct forms of flowers, the relation is always that which exists between the two forms of primula. On the contrary, in many cases, the one form is specially adapted to be fertilised by the agency of insects; while the other is so constituted as to be self-fertile. The latter type of flower is generally smaller than the former, and indeed in some cases, to which attention was first called by Kuhn, and which he appropriately named cleistogamic, they are so much reduced that they would scarcely be recognised as flowers at all. The complexity introduced by these interesting adaptations seems to reach its climax in Oxalis sensitiva, which bears no less than six kinds of flowers.
Self-fertilising flowers have, of course, the great advantage that in them fertilisation is more probable, and may even be rendered practically certain. This is necessarily a great benefit; but, on the other hand, it is counterbalanced by the fact that, as Mr. Darwin showed in a subsequent work, to which we shall immediately refer, cross-fertilised plants. are more vigorous and healthy. Indeed, though we know of many cases in which self-fertilisation is impossible, there is not one of which the opposite can be predicated.
No group of plants present more complex and beautiful contrivances than the Orchids, to which Mr. Darwin devoted a special work, “On the various Contrivances by which British and Foreign Orchids are Fertilised by Insects.” Many of these are very singular. In Listera ovata, for instance, the moment the insect touches a particular part of the flower, a drop of fluid is instantaneously secreted, which glues the pollen to the head of the insect. In Cypripedium the flower is so arranged that any bee which is once tempted into the hollow, shoe-like receptacle, from which the flower derives its name, is compelled to make its exit by a narrow passage, in passing through which it first of all rubs its head against a sticky surface, which renders it adhesive, and then against the stamen, from which it thus carries off a certain quantity of pollen. In Cephalanthera, when the flower is mature, the terminal portion of the labellum turns downwards, so as to form a sort of doorway through which insects can obtain access, and thus fertilise the flower. As soon as this object is effected the labellum rises again and shuts the triangular
door, thus completely closing the flower, and preventing the access of insects, which would then be useless or even mischievous. In Catasetum, one of the large tropical species, the flower is very large ; one portion of it is highly sensitive, and as soon as it is touched by an insect the flower literally throws its pollen with unerring aim at the visitor. With such force is this effected, that the pollen may be thrown three or four feet.
It would be impossible, of course, within the limits of the present article, to allude, however briefly, to all of these beautiful cases.
But though Mr. Darwin and other botanists bave succeeded in throwing much light on the peculiar and beautiful structures presented by orchids and other flowers, there are still many problems which remain to be solved, even amongst our English species. The Bee Ophrys, for instance, seems to be specially adapted for self-fertilisation, and insects very rarely visit the flowers. Mr. Darwin himself has never seen such a case. Robert Brown even supposed that the flowers resembled bees, in order to deter insects from visiting them. But though the conclusion arrived at by Mr. Darwin is probably the safest, that under unknown circumstances, and perhaps at very long intervals of time, one individual Bee Ophrys is crossed by another, still the case seems to be very puzzling.
Malaxis paludosa seems to offer strong evidence in favour of Mr. Darwin's general views as to the origin of species by natural selection. In the normal flower, from which the Orchids may be assumed to have descended, the part which is known as the labellum is properly directed upwards; but in the majority of the Orchids, it has become desirable that it should be at the lower side of the flower, and this is effected by a partial twisting of the ovary. In Malaxis paludosa, on the contrary, the labellum is directed upwards, and this is effected by an additional twist being given to the ovary ; whereas, if the plant had been directly created, one can see no reason why the ovary should be twisted at all. It would, of course, be a much simpler arrangement that each flower should be fertilised by its own pollen. These arrangements, however, presuppose that "cross-fertilisation" is an advantage, and Mr. Darwin's work, “The Effect of Cross and Self Fertilisation in the Vegetable Kingdom,” is devoted to showing, which he does by the most careful and conclusive experiments, that plants produced from cross-fertilised flowers are really more vigorous than those from self-fertilised ones. In Ipomæa, for instance, the general average of a number of experiments gives the size of the crossed as compared with that of the self-fertilised plants as 100 to 77, and in fertility as 100 to 51. If an equal number of both were grown together in a pot, where there was not room for all, the cross-fertilised plants tended to crowd out and smother the others. The cross-fertilised plants also seemed to be more capable of resisting cold. The action of insects, therefore, not only renders our flowers beautiful, but more fertile, vigorous, and healthy likewise.
It is very curious that crosses between flowers grown on cuttings from the same plant, do not give any beneficial result.
In by far the majority of cases, the relation between flowers and insects is one of mutual advantage. In his work, however, on “Insectivorous Plants," Mr. Darwin deals with a variety of interesting species, in which we meet with a very different state of things. The first observation on insect-eating flowers was made about the year 1768, by our countryman Ellis, in Dionæa, a North American plant, the leaves of which have a joint in the middle, and thus close over, seize, and actually digest any insect which may alight on them. The plant has recently been studied by an American botanist, Mr. Canby, who fed it with various substances, with different results. He found that cheese, for instance, disagrees horribly with the leaves, turning them black, and finally killing them.
In the summer of 1860, Mr. Darwin's attention was attracted by the large number of insects caught on the leaves of our common sundew (Drosera rotundifolia), which are covered with glutinous glandular hairs or tentacles-on an average about two hundred on a full-sized leaf. The glands are each surrounded by a drop of an exceedingly viscid solution, which, glittering in the sun, has given rise to the name of the plant. If any object be placed on the leaf, these glandular hairs slowly fold over it, but if it be inorganic, they soon unfold again. On the other hand, if any small insect alights on the leaf, it becomes entangled in the glutinous secretion, the glands close over it, their secretion is increased, and they literally digest their prey. Mr. Francis Darwin has recently shown that plants supplied with insects grow more vigorously than those not so fed. It is very curious, that while the glands are so sensitive, that even an object weighing only 78740th of a grain placed on them is sufficient to cause motion, yet they are “insensible to the weight and repeated blows of drops” of even heavy rain.
Another genus of insect-eating plants is Pinguicula, which frequents moist places, generally on mountains. The leaves are concave, with incurved margins, and the upper surfaces are covered with two sets of glandular hairs. In this case, the naturally incurved edges curve over still more, if a fly or other insect be placed on the leaf.
Another English insectivorous plant is Utricularia, an aquatic species, bearing a number of small bladders or sacs, which have been supposed to act as floats. Branches, however, which bear no bladders, floa just as well as the others, and there seems no doubt that their real use is to capture small aquatic animals, which they do on a large scale The bladders, in fact, are on the principle of an eel-trap, having an entrance closed with a flap, which permits an easy entrance, but effectually prevents the exit of the unfortunate victim. We have not space to refer to Mr. Darwin's other works, such as