But it may be asked, Why have all these unnecessary divisions been maintained up till the present day? Why have they not long ago been given up, since they can and do only lead to the production of three abortive ova, which are foredoomed to perish? Are they mere vestiges, processes which are in themselves meaningless, but have, so to speak, been maintained by the principle of inertia? This principle is certainly operative in some sense and to some extent even in living nature; a process which has been regularly repeated through a long series of generations does not at once cease to be performed when it is no longer of use to the organism concerned. The eyes of animals which have migrated to lightless depths do not disappear all at once and leave no trace; they degenerate very gradually and only in the course of many generations; and it would thus be quite possible to defend the position that these polar or 'maturation divisions' of the ovum are purely _phyletic reminiscences_ without actual significance. But I cannot agree with this opinion. If it were actually so we should expect that the formation of the polar bodies would not still take place in all cases in almost the same manner, for all rudimentary parts and processes vary greatly; we should expect that in many animal groups the polar divisions would not occur, or perhaps that only half the number would occur. But this is not so; in all multicellular organisms, from the lowest to the highest, two reducing divisions take place, and always in almost the same manner, with the exception of a single category of ova, of which I shall presently have to speak. We shall see later that even in unicellular organisms analogous processes may be observed.