THE (ESTROUS CYCLE IN THE MAMMALIA 59 



centration of sexual seasons during one part of the year may 

 be considered as an approach to a condition of polycestrum ; 

 for, if the oestrous periods were to recur at still shorter intervals 

 than is actually the case, they could be regarded as forming so 

 many dicestrous cycles in one sexual season. So far as I am 

 aware, the ferret does not experience oestrus more than twice 

 annually if allowed to breed. 



The polecat is also monoestrous, but breeds only once a 

 year. Mr. Cocks informs me that in captivity the young of 

 this animal are generally born in the first half of June, and that 

 the gestation period, as in the ferret, is about forty days. 



The stoat, weasel, and pine-marten, in their wild state, are 

 almost certainly monoestrous and breed once a year. In the 

 last-mentioned animal Cocks ^ found that a single oestrus may 

 last a fortnight. The stoat and weasel do not appear to have 

 been bred in captivity. The otter in a state of nature breeds 

 only once a twelvemonth (in winter, as a rule, but young may 

 be born at any season according to Cocks). In captivity, how- 

 ever, oestrus may recur at regular monthly intervals all the 

 year round.^ 



The various species of seals are in all probability monoestrous, 

 and have one Utter of young annually. Some species show 

 an almost perfect rhythmic regularity in the recxirrence of their 

 breeding season. Thus, in the case of the harp seal in the north- 

 east of Newfoimdland, and also in Greenland, according to 

 MiUais,^ the pups are born each year between March 8th and 10th. 



' Cooks, " Note on the Gestation of the Pine-Marten," Proc.Zool. Soc, 1900. 



" Cocks, " Note on the Breeding of the Otter," Proc. Zool. Soc, 1881. 

 Mr. Cocks' subsequent experience concerning which he has been kind enough 

 to write to me, confirms the conclusion that there is no anoestrous period in 

 the otter in captivity. There has been some controversy regarding the 

 breeding of the badger. According to Meale- Waldo the period of gestation 

 is between four and five months ('The Badger: its Period of Gestation,'' 

 Zoologist, 1894), but according to Cocks ( ' The Gestation of the Badger," 

 Zoologist, 1903, 1904), this period may be anything between under five and 

 over fifteen months, for although the sexual season may apparently occur at 

 any time of the year, the young are invariably born within a period limited 

 to six weeks. This extraordinary conclusion is based on a number of 

 observations. Fries (" Uber die Fortpflanzung von Melee taxus," Zool. Anz., 

 vol. iii., 1880) describes the badger's ovum as undergoing a resting stage 

 during which development is at a standstill (cf. roe-deer, p. 47). 



' Millais, loc. cit. 



