Columella regarded the gains from the births as a sufficient motive for encouraging these unions, and thought that mothers should be rewarded for their fecundity; Varro, too, seems to have taken this view.
Such is their fecundity that it has been asserted that one female (probably of P. vestimenti) may in eight weeks produce five thousand descendants.
That chinchillas have not under such circumstances become rare, if not extinct, is owing to their extraordinary fecundity, the female usually producing five or six young twice a year.
The marriage rates in quinquennial periods up to 1905 were 19.6, 18.6, 21.0, 19.8, 15.6, 18.6, 18.6, 18.6, 17.4 and 17.4; the ratio of marriages to the marriageable population was for males (above 16 years) 61.5, for females (above 14) 46.0; the fecundity of marriages seemed to have increased, being about twice as high for foreigners as for natives.
Viability, by which are meant fecundity, longevity and vigour, was low in average.