EXERCISE THE SIXTY-SECOND.
An egg is the common origin of all animals.
“Animals,” says Aristotle,[323] “have this in common with vegetables, that some of them arise from seed, others arise spontaneously; for as plants either proceed from the seed of other plants, or spring up spontaneously, having met with some primary condition fit for their evolution, some of them deriving their nourishment from the ground, others arising from and living on other plants; so are some animals engendered from cognate forms, and others arise spontaneously, no kind of cognate seed having preceded their birth; and whilst some of them are generated from the earth, or putrefying vegetable matter, like so many insects, others are produced in animals themselves and from the excrementitious matters of their parts.” Now the whole of these, whether they arise spontaneously, or from others, or in others, or from the parts or excrements of these, have this in common, that they are engendered from some principle adequate to this effect, and from an efficient cause inherent in the same principle. In this way, therefore, the primordium from which and by which they arise is inherent in every animal. Let us entitle this the primordium vegetale or vegetative incipience, understanding by this a certain corporeal something having life in potentia; or a certain something existing per se, which is capable of changing into a vegetative form under the agency of an internal principle. Such primordia are the eggs of animals and the seeds of plants; such also are the conceptions of viviparous animals, and the worm, as Aristotle calls it, whence insects proceed: the primordia of different living things consequently differ from one another; and according to their diversities are the modes of generation of animals, which nevertheless all agree in this one respect, that they proceed from the vegetal primordium as from matter endowed with the virtue of an efficient cause, though they differ in respect of the primordium which either bursts forth, as it were, spontaneously and by chance, or shows itself as fruit or seed from something else preceding it. Whence some animals are spoken of as spontaneously produced, others as engendered by parents. And these last are again distinguished by their mode of birth, for some are oviparous, others viviparous, to which Aristotle[324] adds a vermiparous class. But if we take the thing as simple sense proclaims it, there are only two kinds of birth, inasmuch as all animals engender others either in actu—virtually, or in potentia—potentially. Animals which bring forth in fact and virtually are called viviparous, those that bring forth potentially are oviparous. For every primordium that lives potentially, we, with Fabricius, think ought to be called an egg, and we make no distinction between the worm of Aristotle and an egg, both because to the eye there is no difference, and because the identity is in conformity with reason. For the vegetal primordium which lives potentially is also an animal potentially. Nor can the distinction which Aristotle[325] made between the egg and the worm be admitted: for he defines an egg to be that “from part of which an animal is produced; whilst that,” he says elsewhere,[326] “which is totally changed, and which does not produce an animal from a part only, is a worm.” These bodies, however, agree in this, that they are both inanimate births, and only animals potentially; both consequently are eggs.
And then Aristotle himself, whilst he speaks of worms in one place, designates them by the name of eggs in another.[327] Treating of the locust, he says,[328] “its eggs become spoiled in autumn when the season is wet;” and again, speaking of the grasshopper, he has these words, “when the little worm has grown in the earth it becomes a matrix of grasshoppers (tettigometra);” and immediately afterwards, “the females are sweeter after coitus, for then they are full of white eggs.”
In this very place, indeed, where he distinguishes between an egg and a worm, he adds:[329] “but the whole of this tribe of worms, when they have come to their full size, are changed in some sort into eggs; for their shell or covering hardens, and they become motionless for a season, a circumstance that is plainly to be seen in the vermiculi of bees and wasps, and also in caterpillars.” Every one indeed may observe that the primordia of spiders, silkworms, and the like, are not less to be accounted eggs than those of the crustacea and mollusca, and almost all fishes, which are not actually animals, but are potentially possessed of the faculty of producing them. Since, then, those creatures that produce actually are called viviparous, and those that produce potentially either pass without any general distinguishing title or are called oviparous and particularly as such productions are vegetal primordia, analogous to the seeds of plants, which true eggs must needs be held to be, the conclusion is, that all animals are either viviparous or oviparous.
But as there are many species of oviparous animals, so must there also be several species of eggs; for every primordium is not alike fit to receive or assume every variety of animal form indifferently. Though we admit, therefore, that eggs in a general sense do not differ, yet when we find that one is perfect, another imperfect, it is obvious that they differ essentially from one another. Perfect eggs are such as are completed in the uterus, where they obtain their due dimensions before being extruded; of this kind are the eggs of birds. Imperfect eggs, again, are such as are prematurely excluded before they are of the full size, but increase after they are laid; of this description are the eggs of fishes, crustacea and mollusca; the primordia of insects, which Aristotle entitles worms, are farther to be referred to this class, as well as the primordia of those animals that arise spontaneously.
Moreover, although perfect eggs are of two colours, in other words, are composed of albumen and vitellus, some are still only of one hue, and consist of albumen alone. In like manner, of imperfect eggs, some from which a perfect animal proceeds are properly so called; such are the eggs of fishes; others are improperly so styled, they engendering an imperfect animal, namely, a worm, grub, or caterpillar, a kind of mean between a perfect and an imperfect egg, which, in respect of the egg or the primordium itself, is an animal endowed with sense and motion, and nourishing itself; but in respect of a fly, moth or butterfly, whose primordium it is potentially, it is as a creeping egg, and to be reputed as adequate to its own growth; of this description is the caterpillar, which having at length completed its growth is changed into a chrysalis or perfect egg, and ceasing from motion, it is like an egg, an animal potentially.
In the same way, although there are some eggs from the whole of which a perfect animal is produced by metamorphosis, without being nourished by any remains of the substance of the egg, but forthwith finds food for itself abroad, there are others from one part of which the embryo is produced, and from the remainder of which it is nourished:—although, I repeat, there are such differences among eggs, still, if we be permitted to conclude on the grounds of sense and analogy, there is no good reason wherefore those that Aristotle calls worms should not be spoken of as eggs; inasmuch as all vegetal principles are not indeed animals actually, but are so potentially, are true animal seeds, analogous to the seeds of vegetables, as we have already demonstrated in the particular instance of the hen’s egg. All animals are, therefore, either viviparous or oviparous, inasmuch as they all either produce a living animal in fact, or an egg, rudiment, or primordium, which is an animal potentially.
The generation of all oviparous animals may therefore be referred to that of the hen’s egg as a type, or at all events deduced from thence without difficulty, the same things and incidents that have been enumerated in connexion with the common fowl being also encountered in all other oviparous animals whatsoever. The various particulars in which they differ one from another, or in which they agree, either generally, or specifically, or analogically, will be subsequently treated of when we come to speak of the generation of insects and the animals that arise equivocally. For as every generation is a kind of way leading to the attainment of an animal form, as one race of animal is more or less like or unlike another, their constituent parts either agreeing or disagreeing, so does it happen in respect of their mode of generation. For perfect nature, always harmonious with herself in her works, has instituted similar parts for similar ends and actions: to arrive at the same results, to attain the same forms, she has followed the same path, and has established one and the same method in the business of generation universally.
Wherefore as we still find the same parts in the perfect or two-coloured egg of every bird, so do we also observe the same order and method pursued in the generation and development of their embryos as we have seen in the egg of the common fowl. And so also are the same things to be noted in the eggs of serpents and of reptiles, or oviparous quadrupeds, such as tortoises, frogs, and lizards, from all the perfect two-coloured eggs of which embryos are produced and perfected in the same manner. Nor is the case very different in regard to fishes. But of the manner in which spiders and the crustacea, such as shrimps and crabs, and the mollusca, such as the cuttlefish and calamary, arise from their eggs; of the conditions also upon which worms and grubs first proceed from the eggs of insects, which afterwards change into chrysalides or aurelias, as if they reverted anew to the state of eggs, from which at length emerge flies or butterflies—of the several respects in which these differ in their mode of generation from an egg, from what we have found in the hen’s egg, will be matter for remark in the proper place.
Although all eggs consisting of yelk and white are not produced and fecundated in the same manner, but some are made prolific through the intercourse of male and female, and others in some other way (as of fishes); and although there is some difference even in the mode in which eggs grow, some attaining maturity within the body of the parent, others continuing to be nourished and to grow when extruded, there is still no reason why an embryo should not be developed in the same precise manner in every egg—always understood as perfect—as it is in the egg of the hen. Wherefore the history which has been given of the evolution of the chick from the hen’s egg may be regarded as applicable to the generation of all other oviparous animals whatsoever, as well as to the inferences or conclusions which may be deduced from thence.