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Another system which has lately been brought forward, and with much ingenuity, is that of appetencies. The principle, and the short account of the theory, is this : Pieces of soft, ductile matter, being endued with propensities or appetencies for particular actions, would by continual endeavours, carried on through a long series of generations, work themselves gradually into suitable forms; and, at length, acquire, though perhaps by obscure and almost imperceptible improvements, an organisation fitted to the action which their respective propensities led them to exert. A piece of animated matter, for example, that was endued with a propensity to fly, though ever so shapeless, though no other we will suppose than a round ball to begin with, would, in a course of ages, if not in a million of years, perhaps in a hundred millions of years (for our theorists, having eternity to dispose of, are never sparing in time), acquire wings. The same tendency to loco-motion in an aquatic animal, or rather in an animated lump which might happen to be surrounded by water, would end in the production of fins : in a living substance, confined to the solid earth, would put out legs and feet ; or, if it took a different turn, would break the body into ringlets, and conclude by crawling upon the ground.
Although I have introduced the mention of this theory into this place, I am unwilling to give to it the name of an atheistic scheme, for two reasons : first, because, so far as I am able to understand it, the origi, nal propensities and the numberless varieties of them (so different, in this respect, from the laws of mechani. cal nature, which are few and simple), are, in the plan itself, attributed to the ordination and appointment of an intelligent and designing Creator : secondly, because, likewise, that large postulatum, which is all
along assumed and presupposed, the faculty in living bodies of producing other bodies organised like themselves, seems to be referred to the same cause ; at least is not attempted to be accounted for by any other. In one important respect, however, the theory before us coincides with atheistic systems, viz. in that, in the formation of plants and animals, in the structure and use of their parts, it does away final causes. Instead of the parts of a plant or animal, or the particular structure of the parts, having been intended for the action or the use to which we see them applied ; according to this theory, they have themselves grown out of that action, sprung from that use. The theory therefore dispenses with that which we insist upon, the necessity, in each particular, case, of an intelligent, designing mind, for the contriving and determining of the forms which organised bodies bear. Give our philosopher these appetencies; give him a portion of living irritable matter (a nerve, or the clipping of a nerve), to work upon; give also to his incipient or progressive forms, the power, in every stage of their alteration, of propagating their like; and, if he is to be believed, he could replenish the world with all the vegetable and animal productions which we at present see in it.
The scheme under consideration is open to the same objection with other conjectures of a similar tendency, viz. a total defect of evidence. No changes, like those which the theory requires, have ever been observed. All the changes in Ovid's Metamorphoses might have been effected by these appetencies, if the theory were true; yet not an example, nor the pretence of an example, is offered of a single change being kpown to have taken place. Nor is the order of generation obedient to the principle upon which this theory is built.
The mammæ* of the male have not vanished by inusitation; nec curtorum, per multa sæcula, Judæorum propagini deest præputium. It is easy to say, and it has been said, that the alterative process is too slow to be perceived; that it has been carried on through tracts of immeasurable time; and that the present order of things is the result of a gradation, of which no human records can trace the steps. It is easy to say this; and yet it is still true, that the hypothesis remains destitute of evidence.
The analogies which have been alleged, are of the following kind: The bunch of a camel, is said to be no other than the effect of carrying burdens; a service in which the species has been employed from the most ancient times of the world. The first race, by the daily loading of the back, would probably find a small grumous tumour to be formed in the flesh of that part. The next progeny would bring this tumour into the world with them. The life to which they were destined, would increase it. The cause which first generated the tubercle being continued, it would go on, through every succession, to augment its size, till it attained the form and the bulk under which it now appears. This may serve for one instance : another, and that also of the passive sort, is taken from certain species of birds. Birds of the crane kind, as the crane itself, the heron, bittern, stork, have, in general, their thighs bare of feathers. This privation is accounted
* I confess myself totally at a loss to guess at the reason, either final or efficient, for this part of the animal frame; unless there be some foundation for an opinion, of which I draw the hint from a paper of Mr. Everard Home (Phil. Transact. 1799, p. 2) vize that the mamme of the fætus may be formed, before the sex is determined,
for from the habit of wading in water, and from the effect of that element to check the growth of feathers upon these parts; in consequence of which, the health and vegetation of the feathers declined through each generation of the animal; the tender down, exposed to cold and wetness, became weak, and thin, and rare, till the deterioration ended in the result which we see, of absolute nakedness. I will mention a third instance, because it is drawn from an active habit, as the two last were from passive habits; and that is the pouch of the pelican. The description which naturalists give of this organ, is as follows : “ From the lower edges of the under chap, hangs a bag, reaching from the whole length of the bill to the neck, which is said to be capable of containing fifteen quarts of water. This bag, the bird has a power of wrinkling up into the hollow of the under chap. When the bag is empty, it is not seen : but when the bird has fished with success, it is incredible to what an extent it is often dilated. The first thing the pelican does in fishing, is to fill the bag; and then it returns to digest its burden at leisure. The bird preys upon the large fishes, and hides them by dozens in its pouch. When the bill is opened to its widest extent, a person may run his head into the bird's mouth; and conceal it in this monstrous pouch, thus adapted for very singular purposes*.” Now this extraordinary conformation is nothing more, say our philosophers, than the result of habit; not of the habit or effort of a single pelican, or of a single race of pelicans, but of a habit perpetuated through a long series of generations. The pelican soon found the conveniency of reserving in its mouth, when its appetite was glutted, the remainder of its prey, which is fish. The
* Goldsmith, vol. vi. p. 52.
fulness produced by this attempt, of course stretched the skin which lies between the under chaps, as being the most yielding part of the mouth. Every distension increased the cavity. The original bird, and many generations which succeeded him, might find difficulty enough in making the pouch answer this purpose: but future pelicans, entering upon life with a pouch derived from their progenitors, of considerable capacity, would more readily accelerate its advance to perfection, by frequently pressing down the sac with the weight of fish which it might now be made to contain.
These, or of this kind, are the analogies relied upon. Now, in the first place, the instances themselves are unauthenticated by testimony; and, in theory, to say the least of them, open to great objections. Who ever read of camels without bunches, or with bunches less than those with which they are at present usually formed ? A bunch, not unlike the camel's, is found between the shoulders of the buffalo; of the origin of which it is impossible to give the account here given. In the second example ; Why should the application of water, which appears to promote and thicken the growth of feathers upon the bodies and breasts of geese, and swans, and other water-fowls, have divested of this covering the thighs of cranes? The third in stance, which
appears to me as plausible as any that can be produced, has this against it, that it is a singue. larity restricted to the species; whereas, if it had its commencement in the cause and manner which have been assigned the like conformation might be expected to take place in other birds, which fed upon fish. How comes it to pass, that the pelican alone was the inventress, and her descendants the only inheritors, of this curious resource ?
But it is the less necessary to controvert the instances