On the distemper, see also Col. Hutchinson on 'Dog Breaking' 1850 page 279.) "varies materially according to the breed, as well as to the size of the animal." Different breeds of dogs are subject in different degrees to various diseases. They certainly become adapted to different climates under which they have long existed. It is notorious that most of our best European breeds deteriorate in India. (1/68. See 'Youatt on the Dog' page 15; 'The Veterinary' London volume 11 page 235.) The Rev R. Everest (1/69. 'Journal of As. Soc. of Bengal' volume 3 page 19.) believes that no one has succeeded in keeping the Newfoundland dog long alive in India; so it is, according to Lichtenstein (1/70. 'Travels' volume 2 page 15.), even at the Cape of Good Hope. The Thibet mastiff degenerates on the plains of India, and can live only on the mountains. (1/71. Hodgson in 'Journal of As. Soc. of Bengal' volume 1 page 342.) Lloyd (1/72. 'Field Sports of the North of Europe' volume 2 page 165.) asserts that our bloodhounds and bulldogs have been tried, and cannot withstand the cold of the northern European forests.]
Seeing in how many characters the races of the dog differ from each other, and remembering Cuvier's admission that their skulls differ more than do those of the species of any natural genus, and bearing in mind how closely the bones of wolves, jackals, foxes, and other Canidae agree, it is remarkable that we meet with the statement, repeated over and over again, that the races of the dog differ in no important characters. A highly competent judge, Prof. Gervais (1/73. 'Hist. Nat. des Mammif.' 1855 tome 2 pages 66, 67.), admits "si l'on prenait sans controle les alterations dont chacun de ces organes est susceptible, on pourrait croire qu'il y a entre les chiens domestiques des differences plus grandes que celles qui separent ailleurs les especes, quelquefois meme les genres." Some of the differences above enumerated are in one respect of comparatively little value, for they are not characteristic of distinct breeds: no one pretends that such is the case with the additional molar teeth or with the number of mammae; the additional digit is generally present with mastiffs, and some of the more important differences in the skull and lower jaw are more or less characteristic of various breeds. But we must not forget that the predominant power of selection has not been applied in any of these cases; we have variability in important parts, but the differences have not been fixed by selection. Man cares for the form and fleetness of his greyhounds, for the size of his mastiffs, and formerly for the strength of the jaw in his bulldogs, etc.; but he cares nothing about the number of their molar teeth or mammae or digits; nor do we know that differences in these organs are correlated with, or owe their development to, differences in other parts of the body about which man does care. Those who have attended to the subject of selection will admit that, nature having given variability, man, if he so chose, could fix five toes to the hinder feet of certain breeds of dogs, as certainly as to the feet of his Dorking fowls: he could probably fix, but with much more difficulty, an additional pair of molar teeth in either jaw, in the same way as he has given additional horns to certain breeds of sheep; if he wished to produce a toothless breed of dogs, having the so-called Turkish dog with its imperfect teeth to work on, he could probably do so, for he has succeeded in making hornless breeds of cattle and sheep.
With respect to the precise causes and steps by which the several races of dogs have come to differ so greatly from each other, we are, as in most other cases, profoundly ignorant. We may attribute part of the difference in external form and constitution to inheritance from distinct wild stocks, that is to changes effected under nature before domestication. We must attribute something to the crossing of the several domestic and natural races. I shall, however, soon recur to the crossing of races.