The same may be said of insects and their egg-laying. This, too, is in many cases only performed once in a lifetime, and the insect dies before it has seen the fruit of its labour. Yet egg-laying is performed in the most effective manner, and with the most perfect security of result. It seems as if the insect knew, so to speak, exactly where, in what numbers, and how it should lay its eggs. Many Mayflies (Ephemeridæ) let their eggs fall all at once into the water in which the larvæ live; many Lepidoptera, such as Macroglossa stellatarum, lay their eggs singly, and on definite plants—the humming-bird hawk-moth, just referred to, on Galium mollugo; others, like Melitæa cinxia, lay their eggs in heaps on the leaves of the way-bread (Plantago media), or, like Aglia tau, on the bark of a large beech-tree. Nothing in these different modes of egg-laying is due to chance or caprice; all is determined and regulated by instinct, and all, as far as we can see, is as well adapted to its purpose as possible. When, for instance, Macroglossa stellatarum lays her eggs singly, or in twos or threes, on the green leaves of the food-plant, it thereby obviates the danger of scarcity of food for the comparatively large caterpillars, since not many of them could subsist together on a single plant of Galium, while Aglia tau can place several hundred eggs on the same beech-tree trunk without having to fear that its caterpillars will not find abundant nourishment. The precision with which the egg-laying instinct works is even greater in other species in which there are more special requirements, e.g. when the eggs have to be laid on the under side of the leaves, as in Vanessa prorsa, or where they have to be cemented together in a little pillar, so that they bear a deceptive resemblance to the green flower-buds of the food-plant (the stinging-nettle).
It is certainly astonishing how exactly the stimulus in these cases is specialized to the liberation of the instinct. In general the smell of the food-plant of the caterpillar is enough for most butterflies, and this attracts the female ready to deposit its eggs, but complete liberation of the instinct is only effected by the visual impression of the under side of the leaf. We cannot but be astonished that there is room for such finely graded nerve-mechanisms in the little brain of a butterfly, and yet it would be easy enough to adduce still more complex instincts connected with oviposition in insects. The large water-beetle, Hydrophilus piceus, lays its eggs on a floating raft made by itself; the gall-wasps must first pierce with their ovipositor into a particular part of a particular plant to be able to lay the eggs in the proper place, and this in no haphazard way but with great carefulness and in a perfectly definite manner. But there is no necessity to refer here to many or to the most complicated cases of egg-laying; I only wish to show that, even in the simple cases, such as that of the butterflies just referred to, there is a precisely regulated combination of actions which is executed mechanically, and which cannot be interpreted as inherited habit, because it never was a habit in any individual of any generation.
It is thus placed beyond the possibility of doubt that very many instincts, at least, must depend on selection, and it would be useless to go further in this direction by extending our survey to other groups of instincts. I shall, however, return later on to the study of instincts, and, after we have become acquainted with the main features of the laws of inheritance, it will then be seen that, even among higher animals, instincts can never be interpreted in terms of the Lamarckian principle.
LECTURE IX
ORGANIC PARTNERSHIPS OR SYMBIOSIS
Hermit-crabs and sea-anemones—Hermit-crabs and hydroid polyps—Fishes and sea-anemones—Green fresh-water polyps—Green Amœba—Sea-anemones and yellow Algæ—Cecropia trees and ants—Lichens—Root fungi—Origin of Symbiosis—Nostoc and Azolla apparently contradict the origin through natural selection.
We have already seen, by means of many examples, to what a great degree animals and plants are able to adapt themselves to new conditions of life; how animals imitate their surroundings in colour and form, how instincts have varied in all directions, how plants have made use of the chance of frequent contact with little animals to obtain nourishment from them, and have developed contrivances adapted for bringing as many of these as possible into their power and causing them to yield them the largest possible amount of food. A great many of these could only be interpreted in terms of natural selection, and in others it seemed at least very probable that selection was one of the factors in bringing them about.
Particularly clear proof of the reality of natural selection is afforded by those cases where one form of life associates itself with a very different one so intimately that they are dependent on one another and cannot live without one another—at least in extreme cases—and that new organs, and, indeed, new dual organisms, are sometimes produced by this interdependence of life. This phenomenon—so-called 'Symbiosis'—was discovered by two sharp-sighted botanists, Anton de Bary and Schwendener. But Symbiosis occurs not only between plants; it occurs also between plants and animals and between two species of animal, and we understand by it a life of partnership depending on mutual benefits, so that each of the two species affords some advantage to the other, and makes existence easier for it. In this respect Symbiosis differs from Parasitism, in which one species is simply preyed upon by another without receiving any benefit from it in return, and also from the more innocent Commensalism of Van Beneden, the table-companionship in which one species depends for its existence on the richly-spread table of another. Symbiosis is particularly interesting, because, in addition to extreme cases with marked adaptations, many occur which are of great simplicity, and which seem to have brought about almost no change in the two associated species.