Though dogs are very ardent in their amours, it does not prevent their duration, for they continue to propagate during life, which is usually limited to fourteen or fifteen years, though some have been known to live till twenty. Length of life in dogs is, like that of other animals, proportioned to the time of his growth: for as they are about two years in coming to maturity, so they live to twice seven. The dog's age may be known by his teeth, which, when he is young, are white, sharp, and pointed; and which, in proportion as he advances in age, become black, blunt, and unequal; it is also to be known by the hair, for it turns grey about the nose, forehead, and round the eyes. These animals, though naturally vigilant, active, and formed for exercise, become, by being over-fed in our houses, so heavy and idle, that they pass their lives in sleeping and eating. This sleep, which is almost continual, is accompanied by dreams, which is perhaps a mild manner of existing; and notwithstanding they are naturally voracious, yet they can subsist without eating a considerable time. In the Memoirs of the Academy of Sciences, there is an account of a bitch, who having been accidentally left in a country-house, subsisted 40 days without any other nourishment than the stuff on the wool of a mattress, which she had torn to pieces. Water seems to be more necessary for them than food, for they drink frequently and very abundantly; and it is even a vulgar opinion that if they want water for a length of time they become mad. It is a circumstance peculiar to them that they seem to make great efforts, and suffer pain in voiding their excrements. This is not occasioned, as Aristotle alleges, from their intestines becoming narrower in approaching the anus; for, on the contrary, it is certain, that in the dog, as in other animals, the great intestines grow bigger as they proceed downwards, and that the rectum is larger than the colon: the dryness of the temperament of this animal is sufficient of itself to produce this effect.
To give a clearer idea of the different kinds of dogs, of their propagation in different climates, and of the mixture of their breeds, I subjoin a kind of genealogical tree, in which all the different varieties may easily be distinguished. The shepherd's dog is the stock or body of the tree. This dog, when transported into the rigorous climates of the north, such as to Lapland, becomes ugly and small, but in Russia, Iceland, and Siberia, where the climate is rather less rigorous, and the people more civilized, he is not only preserved, but even brought to greater perfection. These changes are occasioned solely by the influence of those climates, which produces no great alteration in his form, for in each of them he has erect ears, long and thick hair, and a wild look; he barks also less frequently, and in a different manner from those that in more favourable regions have been brought to greater perfection. The Iceland dog is the only one that has not his ears entirely erect, but which bend or fold a little at their extremities; and Iceland is, of all the northern countries, that which has been most anciently inhabited by half-civilized men.