Improbable as such a crude idea now seems to any person even moderately acquainted with the extraordinary variety and variability of living forms, it nevertheless contained nothing at all likely to contradict the ordinary experience of the everyday observer in the last century. The handful of plants and animals with which he was personally acquainted consisted for the most part of a few large, highly advanced, and well-marked forms, not in the least liable to be mistaken for one another even by the most hasty and casual spectator. A horse can immediately be discriminated by the naked eye from a donkey, and a cow from a sheep, without risk of error; nobody is likely to confuse wheat with barley, or to hesitate between classing any given fruit that is laid before him as a pear or an apple, a plum or a nectarine. Variability seldom comes under the notice of the ordinary passing spectator as it does under that of the prying and curious scientific observer; and when it comes at all, as in the case of dogs and pigeons, roses and hyacinths, it is no doubt set down carelessly on a superficial view as a mere result of human selection or of deliberate mongrel interbreeding. To the eye of the average man, all the living objects ordinarily perceived in external nature fall at once under certain fixed and recognisable kinds, as dogs and horses, elms and ashes, whose limits he is never at all inclined to confound in any way one with the other. Linnæus, the great father of modern scientific biology, had frankly and perhaps unthinkingly accepted this current and almost universal dogma of the fixity and immutability of species. Indeed, by defining a kind as a group of plants or animals so closely resembling one another as to give rise to the belief that they might all be descended from a single ancestor or pair of ancestors, he implicitly gave the new sanction of his weighty authority to the creation hypothesis, and to the prevalent doctrine of the unchangeability of organic forms. To Linnæus, the species into which he mapped out all the plants and animals then known, appeared as the descendants each of a solitary progenitor or of a primitive couple, called into existence at the beginning of all things by the direct fiat of a designing Creator. He saw the world of organic life as composed of so many well-demarcated types, each separate, distinct, and immutable, each capable of producing its like _ad infinitum_, and each unable to vary from its central standard in any of its individuals, except perhaps within very narrow and unimportant limits.