6. It is evident that female fishes have longer lives than males, because females are caught of a greater age than the males; the upper and more forward parts of all animals are larger and stronger, and more firmly built in the male; the hinder and lower parts in the female. This is the case in the human subject, and all viviparous animals with feet: the female is less sinewy, the joints are weaker, and the hairs finer, in those with hair; in those without hair, its analogues are of the same nature; the female has softer flesh and weaker knees than the male, the legs are slighter; the feet of females are more graceful, in all that have these members. 7. All females, also, have a smaller and more acute voice than the males, but in oxen the females utter a deeper sound than the males; the parts denoting strength, as the teeth, tusks, horns, and spurs, and such other parts, are possessed by the males, but not by the females, as the roe-deer has none, and the hens of some birds with spurs have none; the sow has no tusks: in some animals they exist in both sexes, only stronger and longer in the males, as the horns of bulls are stronger than those of cows. BOOK THE FIFTH. CHAPTER I. 1. We have hitherto treated of the external and internal parts of all animals, of their senses, voice, and sleep, with the distinctions between the males and females; it remains to treat of their generation, speaking first of those which come first in order, for they are many, and have numerous varieties, partly dissimilar, and partly like each other. And we will pursue the same order in considering them as we did before in their division into classes; we commenced our consideration by treating of the parts in man, but now he must be treated of last, because he is much more intricate. 2. We shall begin with the testacea, and after these treat of the malacostraca, and the others in the order of their succession. These are the malacia and insects, next to these fishes, both viviparous and oviparous; next to them birds, and afterwards we must treat of animals with feet, whether viviparous or oviparous; some viviparous creatures have four feet, man alone has two feet. The nature of animals and vegetables is similar, for some are produced from the seed of other plants, and others are of spontaneous growth, being derived from some origin of a similar nature. Some of them acquire their nourishment from the soil, others from different plants, as it was observed when treating of plants.