[8] Since this was written for the first edition observations of this process have been considerably increased, and discussions as to the exact interpretation of these are in full tide; we are surrounded by a wealth of new observations, facts, and explanations, without having attained to a consistent and unified theory. Several naturalists, such as Boveri, Häcker, Wilson, and others, have attempted interpretations, but these are in many points contradictory to one another. It is therefore impossible to enter into the question in detail here; further light from new observations must be awaited. So much we may say, however, that it is not chance alone which presides over the re-arrangement of the chromosomes during the reducing divisions; _affinities_ play a part also; there are stronger or weaker attractions between the chromosomes, which aid in determining their relative position to one another. But let us leave these more intimate questions out of account altogether, and turn our attention to the more obvious and less delicate phenomena, and we find that the re-arrangement of ids (Neotaxis) which we have just discussed affords a simple explanation of the generally observed phenomenon of the differences between individuals! Each individual is different from every other, not in the case of Man alone, but in all species in which we can judge of differences, and this is true not only of descendants of different parents, but even of those of the same parents. Of course the differences between two brothers or two sisters do not depend entirely on the hereditary basis, but in part also on external conditions which have affected them from embryonic development onwards. Let us suppose that of two brothers who have sprung from identical germ-cells one becomes a sailor, the other a tailor; it would not surprise us to find them very different in their fiftieth year, one weather-beaten and tanned, the other pale; one muscular, straight, and vigorous, the other weakly and of bent carriage. The same primary constituents develop differently according to the conditions to which they are exposed. But the two brothers will still resemble each other in the features of the face, colour of hair, form of eyes, stature and proportion of limbs, perhaps even in a birthmark, more than any other human beings of their own or any other family, and this resemblance will depend upon the identity of the hereditary primary constituents, on the similar id-combination of the germ-plasm.