Part 16 (2/2)
If, then, any of the species of _Tenthredo_ had been called into primal existence as an egg, it must have been within such a groove as this; and the groove, if carefully examined, would have presented evidences of having been formed and filled by the curious implement of the parent fly.
Those obscure and obscene Insects, the c.o.c.kroach tribe (_Blattadae_), secrete an extraordinary covering for the protection of their eggs.
”Instead of being laid separately, the eggs are, when deposited, enclosed in a h.o.r.n.y case, or capsule, variable in its form in different species but generally of a more or less compressed oval shape, resembling a small bean. There is a longitudinal slit in the margin of the capsule, each side of which is defended by a narrow serrated plate, fitting closely to its fellow. The inside of this egg-case is divided into two s.p.a.ces, in each of which is a row of separate compartments, every one enclosing an egg, so that the whole resembles the pod of some leguminous vegetable. This capsule, with its precious contents, is constantly carried about by the female for a week or a fortnight, and is then fastened, by means of a glutinous fluid, in some safe locality.
The slit of the capsule is strongly coated with cement, so as to be even stronger than the other parts. In this capsule the young larvae are hatched, and immediately discharge a fluid which softens the cement, and enables them to push open the slit, through which they escape; after their exit the slit shuts again so closely, that it appears as entire as before. In some species it would seem that the females themselves liberate their offspring by seizing the capsule when the larvae are fit for escape, and tearing it with the aid of their forelegs from end to end, by which means the enclosed larvae are set at liberty.”[95]
It is impossible to read this description without being reminded of the manner in which the bean or other leguminous seed links itself with a former generation by means of the dehiscent legume, itself a production of the parent plant. And the same reasoning applies to this case, as to the other;--the egg, if the _Blatta_ was created in that stage, would triumphantly show in the pod with which it was covered, a record of past processes.
So, once more, with the immense tribes of solitary Bees, Wasps, and Spheges. I shall mention but one example, from my own experience. It is the Dirt Dauber (_Pelopoeus flavipes_) of North America. The female of this elegant fly, when about to lay her eggs, builds up a tubular nest of cells with fine mud, which she makes by mingling and kneading road-dust with her saliva. Each tube consists of several cells, separated by transverse part.i.tions of the mortar; and in each, before she closes it up, she lays a single egg, which she then covers with spiders which are to const.i.tute the food of the grub when hatched, and to last it during the whole period of its larval growth. Dead spiders would not do, for their bodies would either dry up, or become putrescent long before the young grub could devour them. On the other hand, if a number of these fierce and carnivorous creatures were immured, in health, they would soon destroy one another. To obviate this, the parent-fly ingeniously stings every spider just sufficiently to paralyse, without killing it. Thus nearly a score of living spiders are packed away in a cell scarcely larger than a lady's thimble; and thus they remain fresh and succulent food for the larva, not only till it is ready to begin its eating task, but even to the close of its repast.
I think this a particularly instructive example. The _Pelopoeus_ was indubitably created; for it exists. As indubitably it was created in some stage of its cyclical life-history. If as an imago, then I press the argument from the necessity of its previous metamorphoses. If as a pupa, or a larva, or an egg, each of these conditions of life was entirely pa.s.sed as an inmate of the mud-walled cottage; which, cottage was built and stocked with food by the industry and skill of the parent-fly. The grub could not have lived without the stored spiders; the spiders could not have been stored (_normally_) without the agency of the fly.
In some other instances the connexion between germ and parent is patent to the eye. The beautiful Star-fish, _Cribella_, pa.s.ses through all its infant metamorphoses, changing from an ovum to an Infusory, thence to a Pluteus (or what is a.n.a.logous to it), thence to a Star-fish, all in the marsupium provided for the occasion, by the drawing together of the arms of the patient mother. The female _Brachionus_ carries its deposited eggs attached to the hinder part of its body; and thus we can trace, through their transparent coats, the gradual development of the organs of the embryo,--the coloured eye, the rotatory cilia, the complex mastax,--and even detect the vigorous movements of these and other parts, while yet carried hither and thither by the parent.
[Ill.u.s.tration: FEMALE BRACHIONUS, WITH EGGS.]
But further, in the cla.s.s from which I have taken this last ill.u.s.tration--that of the ROTIFERA--there are examples of viviparous genera; and these, because of the perfect transparency of all the integuments, are peculiarly instructive and germane to my argument.
In _Rotifer macrurus_ the ovary with its germinal vesicles is distinctly seen occupying one side of the animal. From this one of the vesicles enlarges, until it becomes a long-oval translucent sac, nearly filling the whole left side of the visceral cavity. A kind of spasmodic movement is suddenly observed in this oblong ovum, and instantly we see, in its place, a well-developed living young; as distinctly visible as if it were excluded. It lies in a bent position, with its foot upturned; is nearly half the length of the parent; is furnished with a proboscis, with a pair of crimson eyes, with ciliary wheels, with a mastax whose toothed hemispheres frequently work vigorously, and with all the viscera proper to the species.
[Ill.u.s.tration: PREGNANT ASPLANCHNA.
_a._ _Unborn young._]
In the beautiful, comparatively large, and economically singular genus, _Asplanchna_, the same process of development can be watched with perfect facility through every stage.
In the body of the female parent, as transparent as the clearest gla.s.s, the band-like ovary is seen floating in the visceral cavity, with several ova in various degrees of advancement. We trace one of these till it becomes a manifestly living young in the ovisac, lying along at the bottom of the parental cavity, more than one-third of whose volume is occupied by it:--supposing it to be a female infant. All its organs,--the eyes, the jaws, the stomach, the pancreatic glands, _the ovary with its nuclei_, the muscles, the rotatory cilia, &c. can be traced with the utmost distinctness long before birth, and its motions are strong and voluntary.
Neither in this case, nor in that of _Rotifer_, does the young animal pa.s.s through any metamorphosis; the unborn young has the full development of the parent, in every respect but size. In each case, the _visible_ life-history of the individual commences not at birth, but at a period long antecedent, if indeed it can be said to commence at all, where we see it gradually developed from a nucleus, which was an integral part of the parental ovary, _even before that parent's birth_.
In the case of the amusing little Water-fleas (_Daphnia_), we have another example of viviparous generation, which, owing to the same cause as in the ROTIFERA,--the transparency of the integument, can be followed through all its stages by the eye of the observer. The eggs of this little Crustacean are deposited in a special chamber within the valves of the parent, where they are hatched. The young remain in their receptacle for a period, which varies according to the temperature, but long enough for them to undergo important changes in structure, and to pa.s.s their first moult.[96]
Here, again, it is impossible to select a condition which does not take hold of a pre-existence; for the youngest independent stage is dependent on earlier stages; and these are pa.s.sed in visible connexion with the parent.
It is true there is in this genus, another mode of reproduction, by means of eggs which are thrown off enveloped in an organic covering, called the ephippium. If this condition be selected for the argument of my supposed opponent, I reply that it amounts to nearly the same thing; only the case will then come into the category of those animals whose earliest stages are protected by coverings formed from the body of the parent,--like the _Hypogymna_, and the c.o.c.kroach, already alluded to.
Where then, in these species, can we possibly select a stage of life, which is not inseparably and even visibly connected with a previous stage?
If we come to the vertebrate creatures, the argument becomes a.s.suredly not less convincing. The formidable Shark, which we considered as a well-toothed adult ready for slaughter, let us suppose to have been created in the harmlessness of infancy. It is a slender thing, some ten or twelve inches long, bent upon itself, inclosing in the ring thus made, the vitellus or yelk-bag, the contents of which are in process of being absorbed into the abdomen. But the whole,--Shark, yelk-bag, and all--is imprisoned in a brown h.o.r.n.y capsule, that looks like a pillow-case, with long tapes appended to the four corners.
This very peculiar protecting capsule points clearly to a peculiar structure in the parent. The embryo was not inclosed in the pillow-case, at its first formation; but, in the course of its descent from the ovary through the oviduct, it had to pa.s.s a region of the latter, where was a thick glandular ma.s.s,--the nidamental gland,--whose office it was to secrete a dense layer of alb.u.men, with which, the embryo became invested. This substance took the form of the flattened purse, or pillow-case, with produced angles, above described, and on its exclusion from the duct a.s.sumed a very tough h.o.r.n.y consistence, and a dark mahogany colour.
The comparative anatomist would, therefore, without the least hesitation, refer the origin of the investing capsule to the nidamental glands of the female Shark; but supposing the embryo to be but just created, his physiological science would only lead him to a false conclusion.
If the Tree-frog afforded us evidence of pre-existent time, in the metamorphosis which it must naturally have experienced from the tadpole to the reptilian condition, what shall we say to that strange and uncouth member of the same cla.s.s,--the Surinam Toad (_Pipa_)? Little would be gained by selecting the germ-stage, as the presumed epoch of creation in this case; for, according to the extraordinary economy of this genus, the male acts as midwife, and the female as wet-nurse, to the hopeful progeny.
”As fast as the female deposits her eggs, the male who attends her arranges them on her broad back, to the number of fifty or upwards. The contact of these eggs with the skin appears to produce a sort of inflammation; the skin of the back swells, and becomes covered with pits or cells, which enclose each a single egg, the surface of the back resembling the closed cells of a honeycomb. The female now betakes herself to the water; and in these cells the eggs are not only hatched, but the tadpoles undergo their metamorphosis, emerging in a perfect condition, though very small, after a lapse of _eighty-two days_ from the time in which the eggs were placed in their respective pits.”
To a tyro in animal physiology it might seem that the smooth rounded egg of a bird or a lizard, presents an example of an organism in the simplest possible condition, and in a stage which, if any can be, is independent of anything that went before.
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