THE CESTROUS CYCLE IN THE MAMMALIA 69 



necessarily controlled by natural selection, acting in the interests of 

 the next generation. 



The effects of lactation upon the recurrence of cestrus vary widely, 

 and are often different among individuals belonging to the same 

 species. Thus, although the mare as a general rule is capable of 

 becoming pregnant while suckling, in some individuals the sexual 

 season is postponed, the mares only becoming pregnant once in two 

 years. 



The return of menstruation during lactation in women has been 

 dealt with recently by Heil ^ and Dingwall Fordyce.^ Heil, who had 

 studied the conditions of two hundred nursing mothers, expresses 

 the belief that the occurrence of menstruation and not the condition 

 of amenorrhoea is the normal state during lactation, but that 

 menstruation is not so frequent in the later lactations as during the 

 earlier ones. Fordyce has reached similar conclusions, finding that 

 menstruation occurred during lactation in forty per cent, of the cases 

 in which suckling was performed, while in ninety-two per cent, of 

 the cases its return was within nine months of parturition, and that 

 menstruation during lactation was commoner with the earlier than 

 with the later lactations, showing that age is an important factor. 



Amenorrhoea, or .the absence of menstruation, may be due to 

 aufemia or some pathological condition. Fraenkel ^ states that it was 

 common in Germany during the recent war, when it resulted from 

 poverty of nutrition, overwork, and strain. A condition of amenorrhoea 

 is said to occur normally among the Esquimaux during the winter, 

 when it is clearly comparable to the anoestrum of animals. (See also 

 above, p. 60.) 



The histological changes which occur in the internal generative 

 organs of various Mammalia during the oestrous cycle are described 

 at some length in the succeeding chapters. 



' Heil, "Laktation und Menstruation," Monatsschr. f. Oebwrtsh. u. Gynak., 

 vol. xxiv., 1906. 



^ Fordyce, "An Investigation into the Complications and Disabilities of 

 prolonged Lactation." Being an extension of papers published in The Lancet, 

 Pa,rt I., 1906 ; The Brit. Med. Jmir., Part I., 1906 ; and The Brit. Jour, of 

 GhUdren's Diseases, ,1906. Gellhorn ("Abnormal Mammary Secretion," Jour. 

 Amer. Med. Assoc., 2Ist November 1908) mentions a case of an ape {fiercopithecus) 

 in wliich menstruation always disappeared during profuse lactation, but 

 reappeared as soon as the mammary secretion ceased or became markedly 

 decreased. 



' Fraenkel, " Eierstockstatigkeit und KriegsamenorrhSe,'' Zent. f. Gi/n., 

 Jahrgang xli., 1917. 



