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ing is left in the centre of the shell for the entrance of the food: a bony structure, in which five teeth are inserted, fills up this aperture; and as these parts are moveable by numerous muscles, they form a very complete organ of mastication.
Such is the variety which the hand of nature, sometimes as if sportive, but always skilful, has introduced into the structure and arrangement of the teeth of animals, or the organs that are meant to supply their place.
The SKIN AND ITS APPENDAGES offer an equal diversity, and constitute the next subject of our enquiry; intermingling, however, a few connected observations on the OVA.
All living bodies, whether animal or vegetable, are furnished with this integument: in all of them it is intended as a defence against the injuries to which, by their situation, they are commonly exposed; and in most of them it also answers the purpose of an emunctory organ, and throws off from the body a variety of fluids, which either serve by their odour to distinguish the individual, or are a recrement eliminated from its living materials.
This integument accompanies animals and vegetables from their first formation: it involves equally the seed and the egg; and, possessing a nature less corruptible than the parts it encloses, often preserves them uninjured for many years, till they can meet with the proper soil or season for their healthy and perfect evolution.
This is a curious subject, and must not be too hastily passed over. After fish-ponds have been frozen to the very bottom, and all the fishes contained in them destroyed: or after they have been
completely emptied, and cleared of their mud; eels and other fishes have been again found in them, though no attempt has been made to re-stock the ponds. Whence has proceeded this reproduction? Many of the ancient schools of philosophy, and even some of those of more modern date, refer us to the doctrine of spontaneous generation, and believe that they have here a clear proof of its truth. But this is to account for a difficulty by involving ourselves in one of a much greater magnitude. It is a petitio principii of which we stand in no need, and which we should be careful how we concede. The reproduced fishes have alone arisen from the ova of those which formerly inhabited the fish-pond; and which, from some cause or other, had sunk so deep into the soil, as to be beyond the germinating influence of the warmth and air contained in the supernatant water, communicated to it by the sun and the atmosphere. But the indestructible texture of the integument which inclosed the fecundated ova has preserved them, perhaps for years, from injury and corruption; and they have only waited for that very exposure to light, air, and warmth, which the removal of the superior stratum of mud has produced, to awaken from their dormant state into life, form, and enjoyment; and but for which they would have remained in the same state, dormant but not destroyed, for ten or twelve times as long a period.
So, in the hollows upon our waste lands, when they have been for some time filled with stagnant water, we not unfrequently find eels, minnows, and other small species of the carp genus, leeches*,
* See Wild. p. 120. note.
water insects, and wonder how they could get into such a situation. But the mud which has been emptied out of the preceding fish-pond has perhaps been thrown into these very hollows; or the ova of the animals have been carried into the same place by some more recondite cause; and they have been waiting, year after year, for the accidental circumstance which has at length arrived, and given them the full influence of warmth, water, light, and air.
The ova of many kinds are peculiarly light, and almost invisibly minute. They are hence, when the mud, which has been removed from fish-ponds becomes dry and decomposed into powder, swept by the breeze into the atmosphere, from which they have occasionally descended into the large tanks which are made in India as reservoirs for rainwater; and producing their respective kinds in this situation, have appeared, to the astonishment of all beholders, to have fallen from the clouds with the rain itself. Dr. Thomson, in adverting to this curious fact, observes that it is difficult to account for it satisfactorily. The explanation now offered will, if I mistake not, sufficiently meet the case.
Many insects can only be hatched in a particular animal organ; and it is the office of the integument of the ovum to preserve it in a perfect state till it has an opportunity of reaching its proper nidus. Thus the horse-gadfly, or oestrus equi, deposits its eggs on the hairs of this animal, and sticks them to the hair-roots by a viscous matter which it secretes for this purpose. But here they could never be hatched, though they were to remain through the
* Annals of Philos. viii. p. 70.
whole life of the horse: their proper nidus is the horse's stomach or intestines, and to this nidus they must be conveyed by some means or other; and in their first situation they must remain and be preserved, free from injury or corruption, till they can obtain such a conveyance. The integument in which they are wrapped up gives them the protection they stand in need of; and the itching which they excite in the horse's skin compels him to lick the itching part with his tongue; and by this simple contrivance the ova of the gadfly are at once conveyed to his mouth, and pass with the food into the very nidus which is designed for them.
It is the same integument that, by its incorruptibility, preserves the caterpillar during the torpitude of its chrysalid state, while suspended by a single thread from the eaves of an incumbent roof; and which thus enables the worm to become transformed into a butterfly. The larve of the gnat, when approaching the same defenceless state, dives boldly into the water, and is protected by the same indestructible sheath from the dangers of an untried element.
In several species the insect remains in its chrysalid state for many years: the locust, in one of its species at least, the cicada septendecim, appears in numbers once only in seventeen years, and the palmer-worm once only in thirty years; cycles not recognised by the meteorologist, but which are well entitled to its attention: and, through the whole range of their duration, it is the integument we are now speaking of that furnishes the animal with a secure protection.
Whence comes it that plants of distant and
opposite climates (for every climate has its indigenous plants as well as its indigenous animals) should so frequently meet together in the same region? that those which naturally belong to the Cape of Good Hope should be found wild in New Holland? and those of Africa on the coast of Norway? and that the Floras of every climate under the heavens should consociate in the stoves and gardens of our own country? It is the imperishable nature of the integument that surrounds their seeds by which this wonder is chiefly effected. Some of these seeds are provided with little hooks, and fasten themselves to the skins of animals, and are thus carried about from place to place; others adhere by a native glue to the feathers of waterfowls, and are washed off in distant seas; while a third sort are provided by nature with little downy wings, and hence rise into the atmosphere, and are blown about by the breezes towards every quarter of the compass. Of this last kind is the light seed of the betula alba, or birch-tree; which, in consequence, is occasionally seen germinating on the summit of the loftiest rocks and the tops of the highest steeples.* But it is to man himself that this dissemination of plants is chiefly owing. He who in some sort commands nature-who changes the desert into a beautiful landscape-who lays waste whole countries, and restores them to their former fruitfulness-is the principal instrument of enriching one country with the botanical treasures
There is an interesting article on this subject published long since the above was delivered; an account of which may be found in the Journal of Science and the Arts, No. vii. p. S.