In this claim of superiority for the evolution hypothesis, the learned philosopher seems to have been almost oblivious of the fact that he was dealing with animal organisms in two aspects: first, in regard to the method by which individuals of the same species come into existence; and, secondly, in regard to the method by which different species have come into existence. In the first case, regularity of method is evinced by the establishment of a uniform process of procreation and gestation. This process, while retaining throughout the different classes of animals one fundamental and characteristic method, namely, the union of the sexes, is widely varied in respect to the time of gestation, the fœtal development, and the nourishment of the young before and after birth. There is no difficulty whatever in discovering the great reason for which this system of the reproduction of individuals was established. The tie that it makes between parents and offspring, and more especially the tie between the female parent and the offspring, was obviously one grand end for which this system of giving existence to individuals was adopted; and although the instinct which arises out of it is in some species feeble and almost inactive, it rises higher and higher in its power and its manifestations in proportion as the animals rise in the scale of being, until in man it exhibits its greatest force and its most various effects, producing at last pride of ancestry, and affecting in various ways the social and even the political condition of mankind. But how can any corresponding connection between one race of animals and another, or between antecedent and subsequent species, be imagined? The sexual impulse implanted in animals leads to the production of offspring of the same race. The desire for offspring keeps up the perpetual succession of individuals, and love of the offspring insures the protection of the newly born by the most powerful of impulses. But what can be imagined as an analogous impulse, appetite, or propensity which should lead one species to strive after the production of another species? Is it said that the different species are evolved out of one another by a process in which the conscious desires, the efforts, the aspirations of the preceding races play no part? This is certainly true, if there was ever any such process as the evolution of species out of species; and it follows that, in respect to one great moral purpose of a process, there is no analogy to be derived from the regularity and uniformity of the process by which individuals of the same species are multiplied. Moreover, in regard to the latter process, we know that a barrier has been set to its operation; for Nature does not now admit of the sexual union between animals of entirely distinct species, and we have no reason to believe that it ever did admit of it at any period in the geological history of the earth.