I do not wish to contradict this conclusion; indeed, I believe that the constitution of the species becomes more and more constant in the way I have indicated, and that an ever more perfect and stable equilibrium of the whole determinant system is thus brought about. It follows that in the course of generations the diverse determinants of the germ-plasm will vary within a progressively shortened radius, and will thus more and more rarely overstep the limits of the 'variation-playground'--and yet I still believe that this justifiable conclusion tells in favour of my interpretation of the utility of the persistence of amphigony once introduced. Let it be remarked, in the first place, that it is by no means essential to the preservation of a useful institution that it should practically justify its utility in _every_ generation. Although, for instance, the warm winter coat of a species of mammal may be necessary to its survival, it does not disappear at once when a winter happens to occur which is so warm that even individuals with poor pellage can survive. Indeed, several such mild winters might occur in succession, in which there was no weeding-out of the individuals with poor fur, and yet the thickness of the winter fur of the species would not become less fixed, just because this character no longer varies perceptibly in an old-established species which has long been perfectly adapted, and it could only be brought into a state of marked fluctuation very slowly through direct influence on the germ-plasm, or through panmixia. But exactly the same thing is true in regard to the determinants of the reproductive cells, in respect of their adaptation to amphimixis, only very much more emphatically. Before going further, I should like to show that the conclusion we have just deduced from the theory, namely, that the equilibrium of the determinant system of a species increases in stability with the duration of its persistence, holds good not only for the whole system, but for its individual parts, that is, for the individual characters and adaptations. Experience teaches that characters are the more exactly and constantly transmitted the older they are; generic characters are more constant than species-characters, order-characters more persistent than family-characters--this is implied even in their name. But we are able to show even in relation to the characters of a species that those which have been fixed for a long time are most precisely and purely transmitted; that is, that their determinants are least inclined to overstep the limits of the 'variation-playground' either in an upward or downward direction. Two groups of facts prove this: first the observed fact that the very different degree of variability which the different species exhibit is by no means common to all the characters of the species in the same measure; for individual characters may be variable or constant in very different degrees.