By degrees the fecundity improved, and in about twenty years became equal to what it is in Europe.

Viability, by which are meant fecundity, longevity and vigour, was low in average.

Such is their fecundity that it has been asserted that one female (probably of P. vestimenti) may in eight weeks produce five thousand descendants.

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.

Two hundred discourses exist to prove his fecundity, while his versatility is shown by the fact that he could treat the same subject differently on half a dozen occasions.