Part 5
I believe you will agree with me, when you have read and considered the above abstract of Dr. Herold's hypothesis, that in it he substitutes a name for knowledge, talks of a _vis formatrix_ because his assisted eye cannot penetrate to the primordial essence or state of the germes of being, and denies the existence of what he cannot discover[63]. From ancient ages philosophers have done the same, to conceal their own ignorance of causes under a sounding name, when they have endeavoured to penetrate within the veil of the _sanctum sanctorum_, which it is not permitted to vain man to enter. This has occasioned the invention, not only of the term in question, but of many others, as little meriting the appellation of _Signs of ideas_; such as _Plastic Nature_, _Epigenesis_, _Panspermia_, _Idea seminalis_, _Nisus formativus_, &c. But upon this subject you cannot do better than consult what the learned Dr. Barclay has said in his admirable work _On Life and Organization_[64], in which he has placed the inanity, the _vox et præterea nihil_, of such high-sounding terms in their true light. The processes of nature in the formation and development of the _fœtus in utero_, of the chick in the egg, of the butterfly in the caterpillar, we in vain attempt fully to investigate; yet we can easily comprehend that pre-existent germes, by the constant accretion of new matter in a proper state, may be gradually developed, but we find it impossible to conceive how, by the action of second causes, without the intervention of the first cause, the butterfly should be formed in the caterpillar, unless it preexists there as a germe or fœtus. "Is it not clear," asks Dr. Virey in his lively manner, "as Blumenbach and other Physiologists maintain, that there is a formative power, a _nisus formativus_, which organizes the embryo? Admirable discovery!" says he, "which teaches us that the fœtus forms itself because it forms itself! As if you should affirm that the stone falls because it falls[65]!" Had Dr. Herold considered what Bonnet says with as much good sense as modesty, he would never have imagined that his discovering the organs of the butterfly one after the other at certain periods in the caterpillar, was any sound argument against their preexistence and coexistence as germes. "Organs," says that amiable and excellent Physiologist, "that have no existence as to us, exist as they respect the embryo, and perform their essential functions; the term of their becoming visible is that which has been erroneously mistaken for the period of their existence[66]." This has been Dr. Herold's grand error; he mistook the commencement of the _appearance_ of the organs of the butterfly for that of their _existence_, and yet the early appearance of the sexual organs ought to have led him to a conclusion the reverse of that which he has adopted.
Dr. Virey has observed with great truth--that "Every being has a peculiar and unique nature, which would be impossible if the body was composed of parts made at several intervals, and without a uniform power that acts by concert[67]:" and every Physiologist acquainted with the history of insects that undergo a _complete_ metamorphosis will allow, that their developments and acquisition of new parts and organs take place according to a _law_ which regulates the number, kind, and times of them, differing in different species, and which has had an invariable operation, since the first creation, upon every sound individual that has been produced into the world.
In consequence of this law, one species changes its skin only _four_ times, and another _five_ or _six_;--in some cases the first skins shall be covered or bristled with hairs or spines, and the last be naked and without arms;--that which forms the case of the pupæ shall differ in form and substance from the preceding skins, varying in both respects in different species; and finally the butterfly shall invariably follow, when no other change but the last mortal one shall take place. Can this law, so constantly observed, be the result of a _blind_ power? Or are we to suppose that the Deity himself is always at work to _create_ the necessary organs in their time and place? Is it not much more consonant to reason and the general analogy of nature, to suppose that these parts and organs exist in embryo in the newly-hatched caterpillar, and grow and are successively developed by the action of the nutritive fluid? In the pupa of many _Diptera_ the inclosed animal, even under the microscope, appears without parts or organs, like a mere pulp; but Bonnet tells us, that if boiled, all the parts of the pupa appear[68], which proves the preexistence of these parts even when not to be discerned, and that nothing but the evaporation of the fluids in which they swim is wanted to render them visible.
Mr. William MacLeay has with great truth observed: "The true criterion of animal as well as vegetable perfection is the ability to continue the species[69];" and in their progress to this state certain changes take place in the parts and organs of _all_ animals and vegetables: there is, therefore, an analogy in this respect between them; and this analogy also furnishes another argument against Dr. Herold's hypothesis, as we shall presently see. These changes are of three kinds: In the vegetable kingdom, at least in the phænogamous classes, there is a succession of developments terminating in the appearance of the generative organs, inclosed in the flower; in this kind the integuments, or most of them, are usually _persistent_. In insects and other annulose and some vertebrate animals, there is a succession of spoliations, or simultaneous changes of the whole integument, till the animal appears in its perfect form with powers of reproduction; in this kind the integuments are _caducous_.--In man and most of the vertebrate animals there is a gradual action of the vital forces in different organs till they are fitted for reproduction; accompanied, as progress is made to the adult state, by the acquisition of certain organs, &c. as of teeth, horns, pubes, feathers, &c.[70] Let us now consider a little in detail the analogies that appear to exist between the second and the first and third kinds. I shall first consider the latter as the least obvious. That able, judicious, and learned physiologist, Dr. Virey, has pointed out no inconsiderable resemblance between the metamorphosis of the insect, and the changes, which he denominates a metamorphosis by metastasis, to which most vertebrate animals are subject. In them, he observes, a state analogous to the _larva_ state begins at the exclusion of the fœtus from the womb; it is deprived of teeth, and its viscera are only accommodated to milk: in the cornute species the horns are in embryo: the digestive system now preponderates, and the great enjoyment is _eating_. A second state, in a degree analogous to that of _pupa_, commences at the period of dentition--the teeth now produce another modification in the intestinal canal, which becomes capable of receiving and digesting solid food: during this period the vital forces are all tending to produce the perfect state of the animal; and in this state, in man especially, the individual is _educated_ and fitted to discharge the duties of active life. Again, analogous to the _imago_ state is the age of puberty, in which the complete development of the sexual powers takes place in both sexes, and the animal has arrived at its acme, and can continue its kind[71]: now the digestive powers diminish in their activity, and love reigns paramount. When this state is fully attained, no further or higher change is to be expected, and the progress is soon towards decay and the termination of the animal's mortal career. So we see that in fact man and other _mammalia_, though they do not simultaneously cast their skins like the insect; or pass into a state of intermediate repose, before they attain the perfection of their nature, like the caterpillar; have their three _states_, in each of which they acquire new parts, powers, and appetites.
But a more striking analogy has been traced between the insects that undergo a complete metamorphosis and the _vegetable_ kingdom; for though the _primary_ analogy seems to be between the _Polypus_ and the _Plant_, yet the _secondary_ one with the _Insect_ is not by any means remote. There are circumstances to which I shall have occasion hereafter to call your attention, which afford some ground for supposing, that the substance of the insect and the vegetable partakes of the same nature, at least approximates more nearly, than that of the insect and the vertebrate animal; and every one who has observed these little creatures with any attention, will have observed amongst them forms and organs borrowed as it were from the kingdom of Flora; and _vice versâ_ the Botanist, if he makes the comparison, will find amongst his favourite tribes many striking resemblances of certain insects. But the analogy does not stop here; for the butterfly and the plant appear to have been created with a particular reference to each other, both in the epoch of their appearance and the changes that take place in them. Thus, as Dr. Virey has observed, the caterpillar is simultaneous with the leaf of the tree or plant on which it feeds, and the butterfly with the flowers of which it imbibes the nectar[72]. Swammerdam, I believe, was the first who noticed the analogy between the _changes_ of the insect and the vegetable, and has given a table in which he has contrasted their developments, including other animals that undergo a metamorphosis[73]: an idea which has been generalized by Bonnet[74], and adopted and enlarged by Dr. Virey[74]. A state analogous to that of the _larva_ in the insect begins in the plant when it is disclosed from the seed, or springs from its hybernaculum in the bulb, &c., or is evolved from the gemma; integument after integument, often in various forms, as cotyledon, radical, cauline, or floral leaves, expands as the stem rises, all which envelopes incase the true representative of the plant, the fructification, as the various skins do the future butterfly. When these integuments are all expanded, the fructification appears inclosed by the calyx or corolla as the case may be, in which the generative organs are matured for their office--this is the bud, which is clearly analogous to the _pupa_ state of the insect. Next the calyx and corolla expand, the impregnation of the germen takes place, and the seed being ripened, and dispersed by the opening of the seed-vessel or ovary of the plant, the individual dies: thus the _imago_ state of the insect has its representative in the plant. "If we place," says Dr. Virey, "here the egg of the insect, next its caterpillar, a little further the chrysalis, and lastly the butterfly--what is this but an animal stem--an elongation perfectly similar to that of the plant issuing from the seed to attain its blossoming and propagation?"[75]
There being, therefore, this general analogy in their progress to that state in which they can continue their species between every part of animated nature, it holds good, I think, that the same analogy should take place in their developments. If the adult man or quadruped, &c. is evidently an evolution of the fœtus, as from microscopical observations it appears that they are[76], if the teeth, horns, and other parts, &c. to be acquired in his progress to that state are already in him in their embryos, we may also conclude that the butterfly and its organs, &c. are all in the newly-hatched caterpillar. Again, if the blossom and its envelopes are contained in the gemma, the bulb, &c. where they have been discovered[77], it follows analogically that the butterfly and its integuments all preexist in its forerunner.
Perhaps after this view of the objections to Dr. Herold's hypothesis, it will not be necessary to say much with regard to the argument he draws from the change of organs--the loss of some and the acquisition of others--since this may readily be conceived to be the natural consequence of the vital forces tending more and more to the formation of the butterfly, and the withdrawing of their action more and more from the caterpillar; I shall not, therefore, enter further into the question, especially since the change of organs will come more regularly under our notice upon a future occasion.
Winged insects, many branchiopod _Crustacea_, and the Batracian reptiles, have been observed by Dr. Virey to bear some analogy to the _mammalia_, _aves_, &c. in another respect. In leaving their egg, they only quit their first integument, answering to the _chorion_ or external envelope of the human fœtus; they therefore still continue a kind of fœtus, so to speak, more or less enveloped under other tunics, and principally in their _amnios_, or the covering in which the fœtus floats in the _liquor amnii_[78]. This the butterfly does in the pupa case; and its birth from this, under this view, will be the true birth of the animal. In the human subject, the ova upon impregnation are said to pass from the ovary through the Fallopian tube into the uterus. In the insect world, upon impregnation, the eggs pass first from the ovaries into the oviduct, answering to the Fallopian tube, which in them terminates in the _ovipositor_, or the instrument by which the parent animal conveys the eggs to their proper station: there is, therefore, nothing properly analogous to the uterus in the insect, and the substance upon which the larva feeds upon exclusion answers the purpose of a placenta.
After this general view of the most modern theories with regard to the _metamorphosis_ of insects, I shall in the present and some following letters, treat separately of the different states through which these little beings successively pass.
* * * * *
The first of these is the _Egg state_, the whole class of insects being strictly oviparous. Some few tribes indeed bring into the world living young ones, and have on that account been considered as _viviparous_, but incorrectly, for the embryos of none of these are nourished, as in the true viviparous animals, within a uterus by means of a placenta, but receive their development within true eggs which are hatched in the body of the mother. This is proved by the observations of Leeuwenhoek, who found eggs in the abdomen of a female scorpion[79]; and of Reaumur, with regard to the flesh-fly (_Musca carnaria_) and other viviparous flies as they have been called[80]. A similar mode of production takes place in vipers and some other reptiles, which have hence been denominated _ovo-viviparous_, to distinguish them from the true viviparous animals--the class _Mammalia_. By far the larger portion of insects is oviparous in the ordinary acceptation of the term. The ovo-viviparous tribes at present known are scorpions; the flesh-fly and several other flies; a minute gnat belonging to Latreille's family of _Tipulariæ_[81]; some species of _Coccus_; some bugs (_Cimicidæ_)[82]; and most _Aphides_, which last also exhibit the singular fact of individuals of the same species being some oviparous and others ovo-viviparous, the former being longer in proportion than the latter.--Bonnet, however, is of opinion that the eggs of the first are not perfect eggs, but a kind of cocoon, which defends the larva, already formed in some degree, from the cold of winter[83].
When excluded from the body of the mother, or from the egg, as has been before observed, some insects appear nearly in the form of their parents, which, with a very slight alteration, they always retain; others, and the greater number, assume an appearance totally different from that of their parents, which they acquire only after passing through various changes. It is to these last, which have chiefly engaged the attention of Entomologists, that the title of metamorphoses has been often restricted. As, however, those insects which undergo the slightest change of form, as spiders do, undergo _some_ change, and almost all insects cast their skins several times[84] before they attain maturity, Linné and most Entomologists, till very recently, have regarded the whole class as undergoing metamorphoses, and as passing through _four_ different states, viz. the Egg--the Larva--the Pupa--and the Imago.
It is obvious, however, that in ovo-viviparous species _three_ states of their existence only come under our cognizance, as these, being hatched in the body of the mother, come forth first under the form of larvæ. There is even one tribe of insects which presents the strange anomaly of being born in the _pupa_ state. This is the Linnean genus _Hippobosca_ (_Pupipara_ fam. Latr.), to which our forest-fly belongs, the females of which lay bodies so much resembling eggs, that they were long considered as such until their true nature was ascertained by Reaumur (most of whose observations were confirmed by De Geer), who, from their size, which nearly equals that of the parent fly--from their slight motion when first extruded--from spiraculiform points which run down each side of them--and lastly, from their producing not a larva, as all other insects' eggs do, but perfect flies in the winged state--inferred, and doubtless with reason, that they are not real eggs, but pupæ, or larvæ just ready to assume the pupa state, which, however strange it may seem, have passed the egg and larva states in the body of the mother[85].
Insects, therefore, as to their mode of birth, may be divided into--
I. _Ovo-viviparous_, subdivided into--
1. _Larviparous_, coming forth from the matrix of the mother in the state of larvæ, as the Scorpion (_Scorpio_), the Flesh-fly (_Musca_), the Plant-louse (_Aphis_), &c.
2. _Pupiparous_, continuing in the matrix of the mother during the larva state, and coming forth in that of _pupa_, as the Forest-fly (_Hippobosca equina_), the Sheep-louse (_Melophagus ovinus_), the Bat-louse (_Nycteribia Vespertilionis_), &c.
II. _Oviparous_. All other insects.
Our business for the remainder of this letter will be with the latter description of these little animals.
The unerring foresight with which the female deposits her eggs in the precise place where the larvæ, when excluded, are sure to find suitable food; and the singular instruments with which, for this purpose, the extremity of their abdomen is furnished, have been noticed in a former letter[86], and those last mentioned will be adverted to in a future one. I shall now, therefore, confine myself to other circumstances connected with the subject, arranged for the sake of order under several distinct heads, as--their _exclusion_--_situation_--_substance_--_number_--_size_--_figure_-- _colour_--and _period of hatching_.
i. _Exclusion._ The exclusion or extrusion of the impregnated eggs takes place, when, passing from the ovary into the oviduct, they are conducted by means of the ovipositor, in which it terminates, to their proper situation. By far the greater number of insects extrude them _singly_, a longer interval elapsing between the passage of each egg in some than in others. In those tribes which place their eggs in groups, such as most butterflies and moths, and many beetles, they pass from the ovaries usually with great rapidity; while in the _Ichneumonidæ_, _Sphegidæ_, _Œstri_, and other parasitic genera, which usually deposit their eggs singly, an interval of some minutes, hours, or perhaps even days, intervenes between the extrusion of each egg. One remarkable instance of the former mode I noticed in my letter on the _Perfect Societies_ of Insects[87]; another may be cited, to which you may yourself be a witness--I allude to that common moth, vulgarly called the _Ghost (Hepialus Humuli)_, which lays a large number of minute black eggs, resembling grains of gunpowder, and ejects them so fast that, according to De Geer, they may be said to _run_ from the oviduct, and are sometimes expelled with the force of a popgun[88]. A Tetrapterous insect, the genus of which is uncertain, is said, when it is taken, to discharge its eggs like shot from a gun[89]. And a friend of mine, who had observed with attention the proceedings of a common crane-fly (_Tipula_), assured me that several females which he caught projected their eggs to the distance of more than ten inches.
A few _Diptera_ extrude them in a sort of chain or necklace, each egg being connected by a glutinous matter with that which precedes and follows it. In a small species of a genus allied to _Psychoda_ (a kind of midge), which one season was abundant in a window of my house, this necklace is composed of eggs joined by their sides, not unlike those strung by children of the seeds of the mallow[90]. Other _Tipulidæ_ on the contrary extrude their eggs joined end to end, so as to resemble a necklace of oval beads. _Beris clavipes_ and _Sciura Thomæ_, two other flies, produce a chain about an inch long, consisting of oval eggs connected, in an oblique position, side by side; an arrangement very similar prevails in the ribband of eggs which drop from some of the _Ephemeræ_[91].