But we also start with another general proposition, namely, that children differ from their parents, and it is this question of the variation in offspring which must now claim our attention for a moment. By this term we mean to convey the fact that although every child has a real resemblance to its parents or its ancestors, it inevitably and invariably shows differences even if these be more minute than the resemblances. In other words the offspring of a human being, though obviously and necessarily, from the continuity of germ-plasm, it must be another human being, is never exactly similar to any other. Now these variations are many of them present from the very beginning, they take their origin in the germ-plasm of the two germ-cells which form the fertilised ovum. They are, that is to say, many of them germinal in origin. These must be carefully distinguished from such characteristics as are afterwards acquired by the child as the result of its adaptation to the environment in which it passes its existence. It would be beyond the scope of the present work to enter into all the various theories which have been put forward to account for the fact of variation. It will be sufficient for our purpose here if the reader remembers that it is a universal tendency in all living protoplasm to exhibit variations. It is just as universal as is its continuity of likeness. Moreover, in dealing with the highest animals in which the fertilised ovum, from which the embryo springs, is produced by the union of germ-cells from male and female, one may readily understand that the different lines of descent of the male and female germ-cells may well be responsible for the differences exhibited in the offspring. Obviously the fertilised ovum, if it has to give rise to a normal individual, cannot retain _all the characteristics_ which were possibly existent in both the male germ-cells and the female germ-cells. Some of them must be suppressed or got rid of, otherwise there would be too many characters in the resulting offspring. And, as a matter of fact, such a reduction does actually take place in the physical tissue comprising the fertilised cell, and it is probably at this stage that variations take their origin.