The microscope teaches us that no difference can be perceived between one germ and another; it cannot, however, be objected on this account that the determining cause of its ulterior development must be something immaterial, rather than the specific kind of its material constitution. The curves and surfaces which the mathematician conceives, or finds conceivable, are more varied and infinite than the forms of animal life. Let us suppose an infinitely small segment to be taken from every possible curve; each one of these will appear as like every other as one germ is to another, yet the whole of every curve lies dormant, as it were, in each of them, and if the mathematician chooses to develop it, it will take the path indicated by the elements of each segment. It is an error, therefore, to suppose that such fine distinctions as physiology must assume lie beyond the limits of what is conceivable by the human mind. An infinitely small change of position on the part of a point, or in the relations of the parts of a segment of a curve to one another, suffices to alter the law of its whole path, and so in like manner an infinitely small influence exercised by the parent organism on the molecular disposition of the germ {78} may suffice to produce a determining effect upon its whole farther development. What is the descent of special peculiarities but a reproduction on the part of organised matter of processes in which it once took part as a germ in the germ-containing organs of its parent, and of which it seems still to retain a recollection that reappears when time and the occasion serve, inasmuch as it responds to the same or like stimuli in a like way to that in which the parent organism responded, of which it was once part, and in the events of whose history it was itself also an accomplice? {79} When an action through long habit or continual practice has become so much a second nature to any organisation that its effects will penetrate, though ever so faintly, into the germ that lies within it, and when this last comes to find itself in a new sphere, to extend itself, and develop into a new creature—(the individual parts of which are still always the creature itself and flesh of its flesh, so that what is reproduced is the same being as that in company with which the germ once lived, and of which it was once actually a part)—all this is as wonderful as when a grey-haired man remembers the events of his own childhood; but it is not more so. Whether we say that the same organised substance is again reproducing its past experience, or whether we prefer to hold that an offshoot or part of the original substance has waxed and developed itself since separation from the parent stock, it is plain that this will constitute a difference of degree, not kind.