In regard, again, to the supposed "transmutation of species," we are equally warranted in affirming that it is destitute of all experimental evidence, and unsupported even by any natural analogy. As the doctrine of spontaneous generation stands opposed to the maxim that _organic life can be produced only by organic life_, so the doctrine of a transmutation of species stands opposed to the equally certain maxim that _like produces like, both in the vegetable and animal kingdoms_. Cuvier has demonstrated, with reference to the birds and reptiles preserved in Egypt, an entire fixity and uniformity of species, in every, even the least, particular, for at least three thousand years.[48] In the actual course of Nature we see no tendency to change; nay, a barrier seems to have been erected in the constitution of Nature itself to prevent the possible confusion of races by promiscuous intercourse, through that provision which renders the mule incapable of reproduction. No plant has ever been found in a state of transition from a lower to a higher form; no instance has ever been produced of one of the algae being transmuted into the lowest form of terrestrial vegetation; nor of a small gelatinous body developing itself into a fish, a bird, or a beast; nor of an ourang-outang rising into a man.[49] It is true, indeed, that "there is a capacity in all species to accommodate themselves, to a certain extent, to a change of external circumstances, this extent varying greatly according to the species. There may thus arise changes of appearance or structure, and some of these changes are transmissible to the offspring; but the mutations thus superinduced are _governed by constant laws and confined within certain limits_. Indefinite divergence from the original type is not possible, and the extreme limit of possible variation may usually be reached in a short period of time; in short, species have a real existence in Nature, and a transformation from one to another does not exist."[50]