Although the cases already mentioned in which great constancy of species is associated with purely parthenogenetic reproduction do not tell in favour of the accuracy of the view just stated, yet the fundamental idea, that amphigony is an essential factor in the maintenance and even in the evolution of species, is undoubtedly sound. We should certainly find neither genera nor species in Nature if amphigony did not exist; but we cannot simply suppose that amphigony and variation are, so to speak, antipodal forces, the former of which secures the constancy of the species, the latter its transformation. In my opinion, at all events, there is no such thing as a 'tendency' of the protoplasm to vary, although there is a constant fluctuation of the characters--dependent on the imperfect equality of the external influences, especially of nutrition. This certainly results, as far as it takes place within the germ-plasm, in a continual upward and downward variation of the hereditary tendencies, and it would lead to increasing dissimilarity of the individuals were it not that amphigony is continually equalizing the differences by a constantly repeated mingling of individuals. Quetelet and Galton have shown that the tendency of this mingling is towards the establishment of a mean; the characters of Man, such as bodily size, fluctuate about a mean, which at the same time shows the maximum of frequency; and the frequency curve of the various bodily sizes assumes a perfectly symmetrical form, so that the average size is the most frequent, and deviations from it upwards or downwards occur more rarely in proportion to the amount of deviation, the largest and the smallest sizes occurring least frequently. Thus an equalizing of variations by means of amphimixis really exists, and the question we have to ask is, How does it come about? The case is assuredly not the same as that in which equal quantities of red and white wine are mixed to make a so-called 'Schiller.' This is proved even by the fact that the mixture may turn out quite different even when the wines--the two parents--are alike: for the children of a pair are often dissimilar. And while the 'Schiller' cannot be separated again into red wine and white, this happens often in sexual reproduction, and sometimes to such an extent that the grandchild exactly resembles one or other of the grand-parents, as is most clearly proved in the case of plant-hybrids. There is thus a deep-seated difference, depending on the fact that what is mingled in amphigony is not simple but composite, not a simple uniform developmental tendency associated with a simple and definite substance, but a combination of several or many developmental tendencies, associated with several equivalent but different material units. These units are the ids or ancestral plasms, and we have seen how they are not only halved by reducing division, but are also arranged in new combinations in amphimixis.