Although tending to be nocturnal, weasels are almost as active by day as by night. Their young, numbering 4 to 9, are born in a nest in a burrow and as with other members of the Order Carnivora, are blind, and incapable of looking after themselves at the time of birth. In _Mustela frenata_ of Montana, breeding occurs in July and August, and the young are born in the following April and May. Wright (1948A:342) showed that the gestation period could not have been less than 337 days in one individual and that it averaged 279 (205-337) days in 18 instances. Findings of the same author (1942B:109) showed that the embryos are implanted only 21 to 28 days before the young are born. In the preceding part of the "long gestation period, the embryos lie dormant in the uterus as un-implanted blastocysts. The young female weasel [of _M. frenata_] mates when 3 or 4 months old." Consequently, in the spring, all females of this species may produce young (Wright, 1942A:348). The circumboreal species _Mustela erminea_ likewise has been shown to have a delayed implantation of the ova. Each of these two species, _M. frenata_ and _M. erminea_, has only one litter per year; but the weasel, _Mustela nivalis_, of the Old World seems to lack the delayed implantation, in this respect resembling the ferret (subgenus _Putorius_) as it does also in its ability to have more than one litter per year (see Deanesly, 1944). The manner of reproduction in the South American species _M. africana_ and the circumboreal species _M. rixosa_ at this writing is unknown. The genus _Mustela_ includes the true weasels, the ferrets and minks. The ferrets commonly are treated as a subgenus, _Putorius_, along with the Old World polecat. The minks usually are accorded subgeneric distinction under the name _Lutreola_, and the true weasels comprise the subgenus _Mustela_, the three subgenera together, along with some other subgenera which are mostly monotypic, comprising the genus _Mustela_. Considered in this way, the group of true weasels, subgenus _Mustela_, has a geographic range roughly coextensive with that of the genus _Mustela_. This range includes Asia and Europe, Northern Africa, North America and northern South America. Java has its weasel. Australia and nearly all the oceanic islands lack weasels, and the animals are absent from roughly the southern half of Africa and the southern half of South America. Other small mustelids, weasellike in shape and with corresponding habits and dentition, take the place of true _Mustela_ in the southern half of Africa and in the corresponding part of South America.