The order mammalia is the resultant of a primary sex-distinction developed by natural selection; but the gorgeous plumage of the peacock’s tail is a secondary sex-distinction developed by sexual selection. If the peacock’s tail were to increase in size and splendor till it shone like the sun and covered an acre,—if it tended so to increase, we will say,—such excessive sex-distinction would be so inimical to the personal prosperity of that peacock that he would die, and his tail-tendency would perish with him. If the pea-hen, conversely, whose sex-distinction attracts in the opposite direction, not by being large and splendid, but small and dull,—if she should grow so small and dull as to fail to keep herself and her young fed and defended, then she would die; and there would be another check to excessive sex-distinction. In herds of deer and cattle the male is larger and stronger, the female smaller and weaker; but, unless the latter is large and strong enough to keep up with the male in the search for food or the flight from foes, one is taken and the other left, and there is no more of that kind of animal. Differ as they may in sex, they must remain alike in species, equal in race-development, else destruction overtakes them. The force of natural selection, demanding and producing identical race-qualities, acts as a check on sexual selection, with its production of different sex-qualities. As sexes, they perform different functions, and therefore tend to develope differently. As species, they perform the same functions, and therefore tend to develope equally. And as sex-functions are only used occasionally, and race-functions are used all the time,—as they mate but yearly or tri-monthly, but eat daily and hourly,—the processes of obtaining food or of opposing constant enemies act more steadily than the processes of reproduction, and produce greater effect. We find the order mammalia accordingly producing and suckling its young in the same manner through a wide variety of species which obtain their living in a different manner. The calf and colt and cub and kitten are produced by the same process; but the cow and horse, the bear and cat, are produced by different processes. And, though cow and bull, mare and stallion, differ as to sex, they are alike in species; and the likeness in species is greater than the difference in sex. Cow, mare, and cat are all females of the order mammalia, and so far alike; but how much more different they are than similar! Natural selection develops race. Sexual selection develops sex. Sex-development is one throughout its varied forms, tending only to reproduce what is. But race-development rises ever in higher and higher manifestation of energy. As sexes, we share our distinction with the animal kingdom almost to the beginning of life, and with the vegetable world as well. As races, we differ in ascending degree; and the human race stands highest in the scale of life so far.