644 THE PHYSIOLOGY OF REPRODUCTION 



throughout the year. An average winter output of less than 

 thirty eggs he found it useful to denote by one factor (L). This was 

 ascertained to be dominant over the allelomorph (/) which is 

 present in fowls laying- no winter eggs. It is not sex-linked. A 

 second fecundity factor (M) denotes a winter egg-production of more 

 than thirty (even up to ninety) provided that it is present along with 

 the first factor (L). Without it the egg-production is still less than 

 thirty. This second factor (M) is sex-linked since it is carried only 

 by ova which give rise to males. The high egg-production, therefore, 

 is transmitted by the cock whose female offspring manifest it in the 

 next generation.! By acting on this hypothesis Pearl was able 

 substantially to increase the productivity of the Barred Plymouth 

 Eock fowls, and although the theory has been criticised it must be 

 admitted that it is extremely difficult to explain the results in any 

 other way. As to what precise physiological signification is to be 

 attached to a factor for fecundity is a point which we are not yet 

 in a position to discuss, but the conception is not necessarily more 

 difficult than that of any other kind of Mendelian factor. 



Gertain Causes of Sterility 



A detailed account of the various pathological conditions which 

 are capable of inducing sterility is outside the scope of this work. 

 The medical publications dealing with the subject form a very 

 considerable literature,^ while the causes of sterility in animals are 

 discussed, though somewhat unsatisfactorily, in many of the veterinary 

 text-books. It may not be out of place, however, to refer briefly to 

 certain of the conditions which are known to induce sterility in man 

 and also in animals. 



In the case of the male an incapacity to procreate is due either 

 to impotence (i.e. inability to perform the sexual act), or to sterility 

 (using the term in the more restricted sense, implying an absence of 

 spermatozoa). Impotence may result from (1) absence of sexual 

 desire, (2) absence of the power of erection and consequent intro- 

 mission, (3) absence of the power of ejaculating the seminal fluid 



1 Pearl, "The Mode of Inheritance of Fecundity in the Domestic Fowl,". 

 Juw. of Exp. ZooL, vol. xiii., 1912. See also papers in Amer. Nat., vols, xlv., 

 xlvi., xlix., and 1., 1911-16. For Castle's criticism see Amer. Nat., vol. xlix., 

 1915. For work on seasonal distribution in egg-production, see Pearl and 

 Surface, U.S. Dept. of Agric., Biireau of Animal Industry, BulletinWQ, Part 2, 1911. 

 For further experiments, etc., on fecundity in fowls see recent papers by Pearl, 

 Curtis, etc., in Biol. Bull., Jour, of Agric. Research, Arch. f. Eni/widc.-Meeh., and 

 varioup American journals. See also Goodale and M'Mullen, " The Bearing of 

 Batios on Theories of the Inheritance of Winter Egg Production," Jowr. of 

 Exp. ZooL, vol. xxviii., 1919. 



2 Miiller (P.), Die Unfruchtbarkeit der Ehe, Stuttgart, 1885. This work 

 contains a bibliography. 



