524 ON PARTURITION.
the time which is described by these ; also the motions of the moon for these are the common origin of generation to all. For it is in accordance with reason that the cycles of inferior things should follow those of the higher." Nature, then, has decreed that the birth and death of animals should have their period and limit after this manner.
. Just as the birth of animals depends on the course of the sun, and moon, so have they various seasons for copulation and different terms of utero-gestation, these last being longer or shorter according to circumstances. " In the human species alone," says the philosopher in the same part of his works, " is the period of utero-gestation subject to great irregularity. In other animals there is one fixed time, but in man several ; for the human foetus is expelled both in the seventh and tenth months, and at any period of pregnancy between these; more- over, when the birth takes place in the eighth month, it is pos- sible for the infant to live." In the majority of animals there is a distinct season for bringing forth their young ; this is generally found to be in the spring, when the sun returns, but in many species it is in the summer, and in some in the autumn, as is the case with the cartilaginous fishes. Hence it is that animals, as the time of labour approaches, seek their accustomed haunts, and provide a safe and comfortable shelter where they may bring forth and rear their young. Hence, too, the title "bird-winds," applied to those gales which prevail toward the beginning of spring, the word owing its origin to the fact of certain birds at that period of the year availing themselves of these winds to accomplish their migrations. In like man- ner stated seasons are observed by those fishes which congre- gate in myriads in certain places for the purpose of rearing their young. Moreover, in the spring, as soon as caterpillars fall under our notice (their ova, as may be observed by the way, like to invisible atoms, being for the most part carried by the winds, and not owing their origin, as commonly supposed, to spontaneous generation, or to be looked upon as the result of putrefaction), straightway the trees put forth their buds, soon to be devoured by these creatures ; and these in their turn fall victims to birds innumerable, and are carried to the nest as food for the young brood. So constantly does this hold, that whenever strange species of caterpillars fall under
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