Beyond the familiar facts that in ourselves, along with the essential organs of sex there go minor structures and traits distinctive of sex, such as the beard and the voice in man, we have numerous cases in which, along with different sex-organs there go general differences, sometimes immense and often conspicuous. We have those in which (as in sundry parasites) the male is extremely small compared with the female; we have those in which the male is winged and the female wingless; we have those, as among birds, in which the plumage of males contrasts strongly with that of females; and among butterflies we have kindred instances in which the wings of the two sexes are wholly unlike--some, indeed, in which there is not simply dimorphism but polymorphism: two kinds of females both differing from the male. How shall we range these facts with the ordinary facts of inheritance? Without difficulty if heredity results from the proclivity which the component units contained in a germ-cell or a sperm-cell have to arrange themselves into a structure like that of the structure from which they were derived. For the obvious corollary is that where there is gamogenesis there will result partly concurring and partly conflicting proclivities. In the fertilized germ we have two groups of physiological units, slightly different in their structures. These slightly-different units severally multiply at the expense of the nutriment supplied to the unfolding germ--each kind moulding this nutriment into units of its own type. Throughout the process of development the two kinds of units, mainly agreeing in their proclivities and in the form which they tend to build themselves into, but having minor differences, work in unison to produce an organism of the species from which they were derived, but work in antagonism to produce copies of their respective parent-organisms. And hence ultimately results an organism in which traits of the one are mixed with traits of the other; and in which, according to the predominance of one or other group of units, one or other sex with all its concomitants is produced. If so, it becomes comprehensible that with the predominance of either group, and the production of the same sex as that of the parent whence it was derived, there will go the repetition not only of the minor sex-traits of that parent but also of any peculiarities he or she possessed, such as monstrosities. Since the two groups are nearly balanced, and since inheritance is never an average of the two parents but a mixture of traits of the one with traits of the other, it is not difficult to see why there should be some irregularity in the transmission of these monstrosities and constitutional tendencies, though they are most frequently transmitted only to those of the same sex.[37]