They include several species widely distributed throughout South America, extending even to the straits of Magellan, from one of which (C. cutleri of Peru) the guinea-pig is derived.
When an albino mouse, rat, guinea-pig or rabbit is crossed with either a pure self or pure pied-coloured form, the offspring are similar to, though not always exactly like, the coloured parent; provided, of course, that the albino is pure and is not carrying some colour or pattern determinant which is dominant to that of the coloured parent used.
Thus the serum of the white rat, which is immune to anthrax, kills the bacillus; whereas the serum of the guinea-pig, which is susceptible, has no such effect.
Although the guinea-pig is a fertile breeder, the wild species only produce one or two young at a birth, and this but once in a year.
The latter was tamed by the Incas, and is the ancestor of the guinea-pig.
On the other hand, he found the weakened virus could be again strengthened by inoculating a feeble animal such as a guinea-pig a day or two old with it, and then inoculating stronger and stronger animals: an increase in strength was gained with each inoculation, until at last the virus could attack the strongest.
The teats vary in number from a single abdominal pair in the guinea-pig to six thoracico-abdominal pairs in the rats; while in the Octodontidae and Capromyidae they are placed high up on the sides of the body.
The unit of antitoxin in Ehrlich's new standard is the amount requisite to antagonize i oo times the minimum lethal dose of a particular toxin to a guinea-pig of 250 grm.
The caudal vertebrae vary from a rudimentary condition in the guinea-pig to a great size in the jumping-hare and prehensile-tailed porcupines.
