The diversity of races in our domestic species is too well known to make it necessary to insist upon this point. I shall only mention that Darwin reckons 150 distinct races of pigeons, and declares that he is not yet acquainted with all. These _races_ are, moreover, sufficiently different to render a redivision into at least four distinct _genera_ necessary, if they are considered as so many _species_. Among mammals analogous facts are noticed, in the case of the dog. At the Dog Show of 1863, the Society of Acclimatisation, which had been very strict in its rules of admission and only received perfectly pure types, collected no fewer than seventy races of dogs. The greater number, however, belonged to Europe, and to France and England in particular; almost all those of Asia, Africa, and America, were absent from the collection, so that altogether we are justified in assuming that there are at least as many races of dogs as of pigeons. As to morphological differences we need only mention bull-dogs and grey-hounds, beagles and Danish carriage dogs, mastiffs and King Charles’s. It is scarcely necessary to remark that these external differences suggest the idea of corresponding modifications in the skeleton, in the proportion and form of the muscles. Anatomical differences are indeed even greater. For example, the skull of the water-spaniel is proportionately double the size of that of the bull-dog. There are among animals, as among plants, some races which develop slowly, and others which increase in size rapidly. As in plants, fecundity is diminished in some and increased in others. When they are too perfect, that is to say, when they are too far removed from their natural type, animal races, like vegetable races, only propagate with great difficulty, or even not at all. Our ordinary races of sheep only give birth once a year to a single lamb; the “hong-ti” twice a year to two lambs each time. The wild sow only litters once a year with but six or eight young, but when domesticated litters twice a year with from ten to fifteen. Her fecundity is therefore at least tripled. In the Indian pig, derived from the “Aperea,” it is more than seven times as great. In dogs, habits imposed by education, transmitted and strengthened by heredity, finally assume the appearance of so many _natural instincts_ by which races are as nicely characterised as by physical peculiarities. This has been established beyond a doubt by the experiments carried on by Knight during more than thirty years. The mention of the beagle and the pointer will be sufficient to recall the contrast which in many cases exists between these _acquired instincts_. Considered as the relative development of the intelligence, properly so called, the difference between races is also very marked in many cases. From this point of view we need only compare the greyhound and the spaniel.