THE (ESTROUS CYCLE IN THE MAMMALIA 71 



and Haycraft l has shown that there are indications of a similar 

 condition existing in Scotland. On this subject Mayo-Smith 2 

 writes as follows : " The largest number [of births] almost 

 always falls in the month of February . . . corresponding to con- 

 ceptions in May and June. . . . Observations tend to show the 

 largest number of conceptions in Sweden falling in June ; in 

 Holland and France, in May June ; in Spain, Austria, and 

 Italy, in May ; in Greece, in April. That is, the farther south, 

 the earlier the spring and the earlier the conceptions." Other 

 facts of a like kind are recorded by Westermarck, who concludes 

 that primitive Man had an innate tendency towards increased 

 powers of reproduction at the end of spring or beginning of 

 summer, and that this tendency became variously modified 

 under the influence of natural selection in the different human 

 races which subsequently arose. 3 



Finally, it may be pointed out that Westermarck's conclusion 

 which seems a very probable one in view of the evidence 

 which he and others have collected is in no way invalidated 

 by the fact that the human female experiences normally an 

 uninterrupted succession of dioestrous (i.e. menstrual) cycles ; 

 for, as already shown, a similar condition is known to exist in 

 several at least of the lower Primates, with which there is also 

 evidence that in a state of nature the breeding functions are 

 restricted to particular seasons of the year. 4 



1 Haycraffc, "On some Physiological Results of Temperature Variations," 

 Trans. Boy. Soc. Edin., vol. xxix., 1880. 



* Mayo-Smith, Statistics and Sociology, vol. i., New York, 1895. Cf. also 

 van Herwerden, loc. cit. 



3 Mayo-Smith (loc. cit.) points out that sexual periodicity in civilised Man is 

 much obscured by social influences. " One great social influence is the time 

 of marriage. Marriage tends to accumulate about the social festivities of 

 Christmas time, and in Catholic countries especially in the period just before 

 Lent." He suggests that in agricultural districts the concentration about 

 Christmas is due to the leisure following the labours of the autumn. " In 

 cities the births are more evenly distributed, showing that artificial life has 

 overcome the influence of seasons and particular occupations." 



* That is to say that, whereas menstruation goes on at regular intervals all 

 the year round, the procestrous or menstrual periods are only followed during 

 the breeding season by cestri at which it is possible for conception to occur. 

 There are some indications that the sexual instinct among males is also 

 periodic, both in the lower Primates and in the human subject, but the 

 periodicity is not so marked as among females. Havelock Ellis (loc. cit.) has 



