Part 16 (1/2)
In many instances there is stronger, or rather more obvious and ordinarily appreciable, evidence of the link between the present and the past generation, than the physiological dependence. The world of Insects, which, from its immensity, and from the high organic rank of its members, affords us so exhaustless a mine of economical wonders,--is rich in examples to the point. A few of these I shall cite.
The eggs of many Insects are not dropped anywhere, at random; for, as the newly-born young have limited powers of locomotion, and yet are in general able to subsist only on some particular kind of food, it is necessary that their birth should occur in the immediate proximity of such food: and therefore that the egg should be so placed. Now this circ.u.mstance would not be specially noteworthy if the locality selected for the deposition of the egg were the same as that in which the parent insect had been accustomed to find its own private enjoyments: we should reasonably say that the eggs were placed here, because the parents happened to be here. The case, however, is very different.
We never find the egg of the Peac.o.c.k b.u.t.terfly adhering to the leaf of a cabbage, nor that of the Garden White to the leaf of a nettle; but the nettle is invariably selected for the former, and a cruciferous plant for the latter.
Yet there is nothing in the individual wants or likings of the b.u.t.terfly, in either case, to account for this. Both the one and the other flutter through the sunny air, alight to drink the water of some slushy pool, rest on the expanding flowers and probe them for nectar, or suck the exuding juices of an over-ripe fruit. But when did you ever see the gorgeous-eyed Peac.o.c.k feeding on a nettle, or the White on a cabbage? Eagerly as they seek these plants, it is solely for the purpose of depositing their eggs where instinct teaches them their unborn progeny will find suitable food.
Supposing, therefore, we had found the egg of either of these b.u.t.terflies at the moment of its creation, we should a.s.suredly have found it on the nettle or the cabbage (as the case might be); because to suppose it in any other situation would be equivalent to supposing it so placed as that the end of its creation--the life of the species created--would be _ipso facto_ frustrated. But, finding it so, the question naturally arises,--Why here, and not elsewhere? and the only possible answer, on the ground of phenomena, is, Because the parent chose this situation for it. And thus we are inevitably thrown back to an anterior generation, which is equivalent to past time.
Again, if we had seen the egg of the Nut Weevil (_Balaninus nuc.u.m_) just come from the creative hand of G.o.d, we should certainly have found it within the immature soft-sh.e.l.led hazel-nut, because there alone would the grub when hatched meet with ”food convenient for” it. And yet if we had sought (ignorant of the fact of its recent creation) the reason of its being there, our acquaintance with entomology would have pointed to the parent beetle, who, with her jaws placed at the tip of a long slender snout, had bored a tiny hole in the tender sh.e.l.l, and had then projected the egg from her abdomen into the interior.
The eggs of the _Oestridae_--for example, the Worble of the Ox (_Oestrus bovis_) or the Bot of the Sheep (_Oe. ovis_)--would be discovered in no other circ.u.mstances than beneath the skin of the former, and at the edge of the nostrils of the latter. For these are the respective situations in which the egg is always deposited, that of the Worble hatching _in situ_, and forming a superficial abscess in communication with the external air, and that of the Sheep-bot producing a larva which crawls up the nostrils of the poor animal, till it finds a suitable resting-place in the frontal sinuses of the skull. To suppose the egg in any other circ.u.mstances than those which I have mentioned, would be to consign it to certain destruction. Yet does not its presence there bear witness to the eclectic care of the parent Gadfly, whose unerring instinct knew how to seek and select the right position?
If you had set yourself to look for the egg of a _Pimpla manifestator_, a common Cuckoo-fly, where would you have looked for it, but in the fatty tissues of a wild bee's grub, that was lodged in a deep hole in some old post? If you had sought elsewhere, you would surely have been disappointed. And would not its presence there bear testimony to the lengthened ovipositor of the well-known brisk and busy fly, and to its remarkable habits?[93]
The grub of the Pill Chafer or Tumble-dung Beetle (_Phanaeus_) feeds on the ordure of _Mammalia_. And, in order that the newly-hatched young may have a copious supply of food at hand, the parent chafer with its jaws detaches a ma.s.s of recent ordure, which it then rolls over the ground with its hind feet, until it acquires a globular form, and a coating of earth or sand. An egg is then deposited in the centre of the ball, which is rolled into a hole made in the earth to receive it. The coating of earth drying and hardening, keeps the interior of the ma.s.s fresh and moist until the young grub is hatched, when it at once begins to devour its savoury and delicate provision.
It would be vain to search for the egg of a _Cynips_ except within a vegetable gall, or at least within the tissues of a plant that are going to produce one. Take as an example _C. quercus_, which produces the spongy excrescence well known as the common Oak-apple. The female Gall-fly is furnished with an ovipositor in the shape of a very fine curved needle, with which she punctures the tender bark of an oak shoot, lodging an egg in the perforation. Stimulated by some fluid, probably, which is poured into the wound at the same time, the sap forms a peculiar tissue around the egg, swelling into a large ball, on which the young grub begins to feed eagerly, and in which it finds the only nutriment on which it could subsist.
Now, if we had found the egg of a Gall-fly newly created, we should certainly have found it in a gall; and the gall would have afforded us indubitable evidence of the wounding of the vegetable tissues, and of the organ, secretion, and instinct of the tiny fly by which the process had been effected. The evidence would be irresistible, but of course it would be fallacious.
Let us now look at a few examples in which the egg is found in invariable a.s.sociation not merely with something that the parent has found for it, but with something that has proceeded from her, a part of herself.
Of this nature are the eggs of that beautiful, but most cacodious, lace-winged fly, _Chrysopa perla_. If you had seen one of these (or more) at the instant of its creation, you would have seen a tiny oval body placed at the extremity of an elastic footstalk half-an-inch in length, and as fine as a hair, standing erect from the surface of a leaf. This thread is composed of a gummy secretion, evolved in a gland attached to the oviduct of the female Lace-fly. When she deposits an egg, she first exudes a drop of this gum on the surface of a leaf, and then, elevating her abdomen, the viscid substance is drawn out in a thread, which presently hardening in the air, the egg is left at the tip of the filament. An experienced entomologist, on seeing this object, would have no hesitation in declaring the origin of the footstalk to be the gum-gland of the female _Chrysopa_; and yet he would certainly have drawn a false inference in the case that I am supposing.
[Ill.u.s.tration: LACE-FLY AND EGGS.]
Many Spiders enclose their eggs in an envelope, the produce of their own bowels. Take an interesting example, as narrated by the eloquent Mr.
Kirby. ”There is a Spider common under clods of earth (_Lycosa saccata_), which may at once be distinguished by a white globular silken bag, about the size of a pea, in which she has deposited her eggs, attached to the extremity of her body. Never miser clung to his treasure with more tenacious solicitude than this spider to her bag. Though apparently a considerable inc.u.mbrance, she carries it with her everywhere. If you deprive her of it, she makes the most strenuous efforts for its recovery; and no personal danger can force her to quit the precious load. Are her efforts ineffectual? a stupefying melancholy seems to seize her; and, when deprived of this first object of her cares, existence itself appears to have lost its charms. If she succeeds in regaining her bag, or you restore it to her, her actions demonstrate the excess of her joy. She eagerly seizes it, and with the utmost agility runs off with it to a place of security.
”The attachment of this affectionate mother is not confined to her eggs.
After the young spiders are hatched, they make their way out of the bag by an orifice which she is careful to open for them, and without which they could never escape; and then, like the young of the Surinam toad (_Rana pipa_), they attach themselves in cl.u.s.ters upon her back, belly, head, and even legs; and in this situation, where they present a very singular appearance, she carries them about with her, and feeds them until their first moult, when they are big enough to provide their own subsistence.”[94]
I waive the argument derived from the fact of the apparent necessity of the mother's care for the new-born young. But the mother's care is indispensable to the appearance of the young at all; not only because the eggs are the produce of her ovary, but also because the envelope which protects them is the produce of her spinning-glands.
There is a furry moth, by no means uncommon, known to collectors as the Gipsy (_Hypogymna dispar_), the eggs of which require to be protected by an elaborate covering, either from extremes of temperature, from light, or from certain electric conditions of the atmosphere. The protection is afforded at the expense of the hair which clothed the mother herself.
Her ovipositor is furnished with a pair of nippers, by means of which she plucks off her own hairs, and makes with them a flat cus.h.i.+on on the surface of a leaf. On this she deposits her eggs in successive layers; and when the full number is laid, she covers them with a roof of hair, slanting downwards and outwards from an apex, so artfully arranged, like the thatch of a cottage, as effectually to throw off water; each layer of hairs overlapping the preceding, and all preserving the same direction, so that, when finished, the work resembles a smooth and well brushed piece of fur.
If, then, a patch of eggs newly-created had been subjected to our inspection, we should have found them snugly protected by their conical roof of thatch; and when we came to examine the thatch microscopically, we should have found it composed of the hairs of _Hypogymna_. And thus again we should have an indubitable and yet deceptive record of a preceding existence.
The numerous species of the genus _Coccus_, to which we are indebted for cochineal, lac, and other products valuable in commerce, afford me an ill.u.s.tration of my argument, more striking than any of the above. In the case of the lac insect (_C. lacca_), for example, the female resembles a little hemispherical scale on the twig of a tree. At a certain period of her life, a pellucid, glutinous substance begins to exude from the margins of her body, which by and by completely covers it, cementing her firmly to the branch, from which she never afterwards moves. She now proceeds to lay her eggs, which one by one as they are extruded are thrust under her, between her abdomen and the surface of the branch. The result of this is, that when the whole are laid, they occupy pretty nearly the same position in relation to the mother as they did before, with this exception, that the abdominal integuments, which before were beneath them, are now above them, and are in close contact with those of the back, so that both together make a double, but still a thin, arched roof over the heap of eggs, which are thus protected till the hatching of the young, when they eat their way out of their long dead mother.
Let me now make my usual application. You say the _Coccus_ was created not an adult insect, which would involve the prochronic stages of its metamorphosis, but as a germ, that is an egg (for the germ of an insect is an egg, and nothing else): well, here is a batch of Coccus-eggs just created, covered with the scaly roof which is necessary to their existence. But this scale is not a record of the mother, but the mother herself, _a prochronic mother_, of course!
Other genera of this wonderful cla.s.s of animals yield us evidences of a somewhat different character, in the structures which the parents form for the reception of their eggs.
One of the most complex and elaborate pieces of mechanism found in any animal organ is the ovipositor of the Sawflies (_Tenthredinidae_). I cannot here describe it at length; it may suffice to say that it consists of two saw-plates, working separately and in opposite directions, the teeth of which are cut into finer teeth; and two supporting plates, very similar to the saws in shape and appearance. The whole flat side of the saw is, moreover, covered with minute sharp points, which give the action of a rasp to the instrument, in addition to that of saw.
By means of this complicated apparatus the parent fly cuts a groove in the twig of the proper shrub, say, a rose-bush. When it is made, the plates are slightly separated, and an egg is laid in the groove. The saw is now withdrawn, and a frothy secretion is deposited, which appears to be intended, by its hardening, to prevent the growth of the wood from closing upon the egg, before the time of hatching arrives.