When we turn from its omissions to study Darwinism as it is, obviously, in the first place, still, more than forty years since it was given to the world, it remains only an hypothesis, based not upon observation or experiment but speculation. In no single instance, past or contemporary, is one species known to have originated from another. The fact upon which Mr. Darwin primarily relies is that of variation. Undoubtedly amongst both plants and animals the offspring are not mere slavish reproductions of their parents, as if cast in the same mould, but exhibit individual differences, working upon which in domesticated instances, man can by selection produce wonderful varieties, as has already been admitted. But, as M. de Quatrefages says,[191] this tells us no more than that species admit of variation; it does not prove that they are capable of transformation, which is the whole point. Certainly, such transformation has never within our knowledge been effected. No breeder or fancier has succeeded, or can hope to succeed, in producing a new species. Moreover, as was pointed out by a critic whose ability Mr. Darwin himself candidly acknowledged,[192] the range of variability as we find it in any species is strictly limited, and although at first it is easy,--in the case of some few animals or plants,--to make great changes in particular directions, by selective breeding, it becomes more and more difficult as we proceed to continue in the same line. If, for instance, in the case of pigeons, a bird can be produced in six years with head and beak only one-half the size of those whence the process started, are we to say that in twelve years their bulk will be reduced to a quarter, and in twenty-four to an eighth? No one could suppose anything so absurd. Mr. Darwin would answer, that he relies upon the vast periods of geologic time to produce alterations such as we cannot possibly attempt within the few years at our disposal. But, it is replied, no length of time will avail anything for such a purpose, unless there be some force to produce variations in the required direction, to the required extent. Such a force is not proved to exist--all the evidence is against it. Where art is most practised in improvement of breeds, or the obtaining of any peculiarities--as with the speed of racehorses, the size of toy-terriers, or the "points" of prize cattle, it becomes most strikingly apparent that we have reached a limit beyond which species will not vary. And until such a cause as we require is fully proved to exist, its supposed effects cannot be made the basis of scientific argument.