First is the so-called zoölogical argument, based on a comparison of the sexual habits and institutions of animals with those of the lowest races of men. In the outset it is important to observe that the physical differentiation of the sexes is itself a product of the struggle for existence. This important fact is made the starting-point of the argument by which Hellwald[272] finds the elements of the human mother-group and of mother-right in earlier animal experiences. Among the lowest members of the animal kingdom there is no individual distinction of sex. That first makes its appearance when the "artistically constructed organism, in order to sustain itself in the process of evolution, is called upon to perform a wider series of functions." Thus "when an animal is forced to greater exertion, when it must work in order to exist, when unresistingly it can no longer suffer the stream of events to press upon it, but withstands it and seeks in it to follow its own course, then the separation of the sexes appears, and, indeed, as a division of labor created by nature for the purpose of developing species." With further evolution, male and female characteristics become more pronounced, in response to the special functions which each sex is called upon to perform. The same process continues in the case of man. To see in him anything other than the "highest and foremost representative of the animal world, one must be drunk with metaphysical nectar, and nothing is better fitted than comparative physiology to humble one's pride in this regard." For man's entire physical organization is "homologous to that of the higher species of animals." Accordingly, the lower a group of men stands on the ladder of culture, the less marked is the "bodily differentiation of the sexes." Among various backward peoples there is relatively slight difference in outward appearance between the men and the women.[273] The growth of sexual variation in physical structure keeps pace exactly with progress in civilization. This progress depends mainly on two original forces. Of these "without doubt the mightiest is hunger," the need of nourishment. For everywhere on earth the "first thought and striving" of living beings is the "stilling of hunger." Next to the struggle for food, the sexual and pairing impulse is the most potent factor in the genesis of society. The former influence, it is important to observe, is the more constant and the more imperative. The latter grows and becomes more acute with increase in refinement and the consequent development of the nervous system.[274] It follows that in the origin of social institutions the erotic or pairing impulse, however important, is a less cogent genetic force than the economic necessity of a food supply. [272] _Die mensch. Familie_, 4 ff.