At any rate, it is not correct to say, with Dr. Gruenhagen, that “the general wedding-feast is spring, when awakening nature opens, to most animals, new and ample sources of living.”[125] This is certainly true of Reptiles and Birds, but not of Mammals; every month or season of the year is the pairing season of one or another mammalian species.[126] But notwithstanding this apparent irregularity, the pairing time of every species is bound by an unfailing law; it sets in earlier or later, according as the period of gestation lasts longer or shorter, so that the young may be born at the time when they are most likely to survive. Thus, most Mammals bring forth their young early in spring, or, in tropical countries, at the beginning of the rainy season; the period then commences when life is more easily sustained, when prey is most abundant, when there is enough water and vegetable food, and when the climate becomes warmer. In the highlands, animals pair later than those living in lower regions,[127] whilst those of the polar and temperate zones generally pair later than those of the tropics. As regards the species living in different latitudes the pairing time comes earlier or later, according to the differences in climate.[128] Far from depending upon any general physiological law, the rut is thus adapted to the requirements of each species separately. Here again we have an example of the powerful effects of natural selection, often showing themselves very obviously. The dormouse (_Muscardinus avellanarius_), for instance, that feeds upon hazel-nuts, pairs in July, and brings forth its young in August, when nuts begin to ripen. Then the young grow very quickly, so that they are able to bear the autumn and winter cold.[129] There are, however, a few wild species, as some whales,[130] the elephant,[131] many Rodents,[132] and several of the lower monkeys,[133] that seem to have no definite pairing season. As to them it is, perhaps, sufficient to quote Dr. Brehm’s statement with reference to the elephant, “The richness of their woods is so great, that they really never suffer want.”[134] But the man-like apes do not belong to this class. According to Mr. Winwood Reade, the male Gorillas fight at the rutting season for their females;[135] Dr. Mohnike, as also other authorities, mentions the occurrence of a rut-time with the Orang-utan.[136] And we find that both of these species breed early in the season when fruits begin to be plentiful,—that is, their pairing time depends on the same law as that which prevails in the rest of the animal kingdom.