From the evidence at hand this much seems sure, that the paternal and maternal chromosomes respectively carry substances, be they ferments, nutritive materials or what not, that are instrumental in giving the final parity of personal characters which we observe to be equally heritable from either line of ancestry. It is clear that most of the characters of an adult organism can not be merely the outcome of any unitary substance of the germ. Each is the product of many cooperating factors and for the final outcome any one cooperant is probably just as important in its way as any other. The individual characters which we juggle to and fro in our breeding experiments seem apexed, as it were, on more fundamental features of organic chemical constitution, polarity, regional differentiation, and physiological balance, but since such individual characters parallel so closely the visible segregations and associations which go on among the chromosomes of the germ-cells it would seem that they, at least, are represented in the chromosomes by distinctive cooperants which give the final touch of specificity to those hereditary characters which can be shifted about as units of inheritance. =Sex and Heredity.--=Whatever the origin of fertilization may have been in the world of life, or whatever its earliest significance, the important fact remains that to-day it is unquestionably of very great significance in relation to the phenomena of heredity. For in all higher animals, at least, offspring may possess some of the characteristics originally present in either of two lines of ancestry, and this commingling of such possessions is possible only through sexual reproduction. As has already been seen, in the pairing of chromosomes previous to reduction, the corresponding members of a pair always come together so that in the final segregation each gamete is sure to have one of each kind although whether a given chromosome of the haploid set is of maternal or paternal origin seems to be merely a matter of chance. Thus, for instance, if we arbitrarily represent the chromosomes of a given individual by _ABC_ _abc_, and regard _A_, _B_ and _C_ as of paternal and _a_, _b_ and _c_ as of maternal origin, then in synapsis only _A_ and _a_ can pair together, _B_ and _b_ and _C_ and _c_, but each pair operates independently of the other so that in the ensuing reduction division either member of a pair may get into a cell with either member of the other pairs. That is, the line up for division at a given reduction might be any one of the following, ABC/abc ABc/abC Abc/aBC AbC/aBc. This would yield the following eight kinds of gametes, _ABC_, _abc_, _ABc_, _abC_, _Abc_, _aBC_, _AbC_, _aBc_, each bearing one of each kind of chromosome required to cover the entire field of characters necessary to a complete organism. And since each sex would be equally likely to have these eight types of gametes and any one of the eight in one individual might meet any one of the eight of