Proceeding upon these breeding maxims, it is evident that we might go on till the heads and legs were evanescent quantities, but this is so palpable an absurdity that we may be quite sure that the premises are not just and that there really is a limit, though we cannot see it or say exactly where it is. In this case, the point of the greatest degree of improvement, or the smallest size of the head and legs, may be said to be undefined, but this is very different from unlimited, or from indefinite, in Mr Condorcet's acceptation of the term. Though I may not be able in the present instance to mark the limit at which further improvement will stop, I can very easily mention a point at which it will not arrive. I should not scruple to assert that were the breeding to continue for ever, the head and legs of these sheep would never be so small as the head and legs of a rat.
It cannot be true, therefore, that among animals, some of the offspring will possess the desirable qualities of the parents in a greater degree, or that animals are indefinitely perfectible.
The progress of a wild plant to a beautiful garden flower is perhaps more marked and striking than anything that takes place among animals, yet even here it would be the height of absurdity to assert that the progress was unlimited or indefinite.
One of the most obvious features of the improvement is the increase of size. The flower has grown gradually larger by cultivation. If the progress were really unlimited it might be increased ad infinitum, but this is so gross an absurdity that we may be quite sure that among plants as well as among animals there is a limit to improvement, though we do not exactly know where it is. It is probable that the gardeners who contend for flower prizes have often applied stronger dressing without success. At the same time it would be highly presumptuous in any man to say that he had seen the finest carnation or anemone that could ever be made to grow. He might however assert without the smallest chance of being contradicted by a future fact, that no carnation or anemone could ever by cultivation be increased to the size of a large cabbage; and yet there are assignable quantities much greater than a cabbage. No man can say that he has seen the largest ear of wheat, or the largest oak that could ever grow; but he might easily, and with perfect certainty, name a point of magnitude at which they would not arrive. In all these cases therefore, a careful distinction should be made, between an unlimited progress, and a progress where the limit is merely undefined.
It will be said, perhaps, that the reason why plants and animals cannot increase indefinitely in size is, that they would fall by their own weight. I answer, how do we know this but from experience? -- from experience of the degree of strength with which these bodies are formed. I know that a carnation, long before it reached the size of a cabbage, would not be supported by its stalk, but I only know this from my experience of the weakness and want of tenacity in the materials of a carnation stalk. There are many substances in nature of the same size that would support as large a head as a cabbage.
The reasons of the mortality of plants are at present perfectly unknown to us. No man can say why such a plant is annual, another biennial, and another endures for ages. The whole affair in all these cases, in plants, animals, and in the human race, is an affair of experience, and I only conclude that man is mortal because the invariable experience of all ages has proved the mortality of those materials of which his visible body is made:
What can we reason, but from what we know?
Sound philosophy will not authorize me to alter this opinion of the mortality of man on earth, till it can be clearly proved that the human race has made, and is making, a decided progress towards an illimitable extent of life. And the chief reason why Iadduced the two particular instances from animals and plants was to expose and illustrate, if I could, the fallacy of that argument which infers an unlimited progress, merely because some partial improvement has taken place, and that the limit of this improvement cannot be precisely ascertained.
