of _N_ are never selected out, and they must gradually give way in the struggle with the neighbouring determinants which are necessary to the species, becoming gradually weaker and ultimately disappearing. This process can, of course, no more be proved mathematically than any other biological processes. No one who is unwilling to accept germinal selection can be compelled to do so, as he might be to accept the Pythagorean propositions. It is not built up from beneath upon axioms, but is an attempt at an explanation of a fact established by observation--the disappearance of disused parts. But when once the inheritance of functional modifications has been demonstrated to be a fallacy, and when it has been shown that, even with the assumption of such inheritance, the disappearance of parts which are only _passively_ useful, and of any parts whatever in sterile animal forms, remains unexplained, he who rejects germinal selection must renounce all attempt at explanation. It is the same as in the case of personal selection. No one can demonstrate mathematically that any variation possesses selection value, but whoever rejects personal selection gives up hope of explaining adaptations, for these cannot be referred to purely internal forces of development. The total disappearance of a part which has become useless takes place with exceeding slowness; the whales, which have existed as such since the beginning of the tertiary period, have even now not completely lost their hind-limbs, but carry them about with them as rudiments in the muscular mass of the trunk, and the birds, which are even older, still show in their embryonic primordia the five fingers of their reptilian forefathers, although even their bird-ancestors of the Jurassic period, if we may argue from _Archæopteryx_, had only three fingers like our modern birds. A long series of similar examples might be given, and modern embryology in particular has contributed much that, like this example of birds' fingers, points to a certain orderliness in the disappearance of the individual parts of an organ which has become superfluous. Parts which, in the complete animal, have disappeared without leaving a trace, appear again in each embryonic primordium, and disappear in the course of the ontogeny. Speaking metaphorically, we might express this on the basis of the determinant theory, by saying that the determinants, as they become weaker, can only control an increasingly short period of the whole ontogeny of the organ, so that ultimately nothing more than its first beginning comes into existence. But this is only a metaphor; we cannot tell what really happens as long as we are ignorant of the physiological rôle of the determinants, and even of the laws governing the degeneration of a useless organ. In respect of the latter, much might still be achieved if comparative anatomy and embryology were studied with this definite end in view, and