2i8 THE EVOLUTION OF SEX. 



in the same paper, that at a temperature of 25 to 26 C, a single 

 Stylonichia would in four days have a progeny of a million, in six 

 days of a billion, in seven and a half days of a hundred billions! In 

 six days the family would weigh one kilogramme, and in seven and a 

 half days one hundred kilogrammes. 



The action of heat may be twofold; up to a certain limit it quickens 

 development and the general life, favoring asexual reproduction and 

 parthenogenesis rather than the sexual process; beyond that limit of 

 comfortable warmth, so variable for different animals, it may induce 

 a feverish habit of body, and hasten reproductive maturity and sexual 

 reproduction. In other words, heat may in some cases favor ana- 

 bolism, in others katabolism. It is intelligible enough to find increased 

 heat sometimes associated with increased asexual reproduction, some- 

 times with accelerated sexuality. Instances of both may be gathered 

 from Semper' s " Animal Life," the classical work on the influence 

 of the environment upon the organism. 



Maupas supplies another vivid illustration of a yet more important 

 enviromental influence, that of food. In another ciliated infusorian 

 {Lcucophrys), so long as food is abundant, fission obtains; but when 

 food grows scanty, there is a metamorphosis without encystation, 

 followed by six successive divisions. These are effected, however, 

 " without vegetative growth, and have for their final object not multi- 

 plication but conjugation." In other words, abundant food is asso- 

 ciated with asexual reproduction; a check to the nutrition brings 

 about the sexual process. Maupas gives a vivid numerical statement 

 of the stimulus to reproduction by a sudden check to the nutrition. 

 Leucophrys at a temperature of 20 C. , in richly nutritive conditions, 

 will give rise to sixteen thousand three hundred and eighty-four indi- 

 viduals in three days; but if the food be then suppressed, this large 

 number will in a few hours be multiplied by sixty-four, resulting in 

 a total of one million forty-eight thousand five hundred and seventy- 

 six individuals! 



From cases already cited, which may be multiplied by consulting 

 Semper' s " Animal Life," supplemented by a summary of more 

 recent researches by one of ourselves, the general conclusions may be 

 drawn, — (a) that heat increases reproduction, either directly or as 

 the result of a preliminary acceleration of growth; (b) that increased 

 food will, of course, favor growth, but reproduction may follow all 

 the more markedly as an exaggerated nemesis; (V) that checks to 

 nutrition, especially in the form of sudden scarcity, will favor sexual- 

 reproduction. The clearest result of all is that a sudden katabolic 

 change favors reproduction, especially in its sexual form.' Anabolic 

 conditions favor reproduction indirectly; the reverse conditions have 



