The constancy of the laws of nature and of effects and causes is the foundation of all human knowledge, though far be it from me to say that the same power which framed and executes the laws of nature may not change them all ’in a moment, in the twinkling of an eye.’  Such a change may undoubtedly happen.  All that I mean to say is that it is impossible to infer it from reasoning.  If without any previous observable symptoms or indications of a change, we can infer that a change will take place, we may as well make any assertion whatever and think it as unreasonable to be contradicted in affirming that the moon will come in contact with the earth tomorrow, as in saying that the sun will rise at its usual time.

With regard to the duration of human life, there does not appear to have existed from the earliest ages of the world to the present moment the smallest permanent symptom or indication of increasing prolongation.  The observable effects of climate, habit, diet, and other causes, on length of life have furnished the pretext for asserting its indefinite extension; and the sandy foundation on which the argument rests is that because the limit of human life is undefined; because you cannot mark its precise term, and say so far exactly shall it go and no further; that therefore its extent may increase for ever, and be properly termed indefinite or unlimited.  But the fallacy and absurdity of this argument will sufficiently appear from a slight examination of what Mr Condorcet calls the organic perfectibility, or degeneration, of the race of plants and animals, which he says may be regarded as one of the general laws of nature.

I am told that it is a maxim among the improvers of cattle that you may breed to any degree of nicety you please, and they found this maxim upon another, which is that some of the offspring will possess the desirable qualities of the parents in a greater degree.  In the famous Leicestershire breed of sheep, the object is to procure them with small heads and small legs.  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.