THEORY OF REPRODUCTION. 235 



tion, or between growth and multiplication, has been as it were 

 the refrain of the preceding pages. This "organic see-saw" 

 is determined by the very constitution of the organism ; in other 

 words, it expresses the fundamental characteristic of living 

 matter. It is an incomplete conception, however, unless it be 

 remembered that about this " organic see-saw " there blows the 

 wind of the environment, swaying it now to one side, now to 

 the other. It is important therefore to illustrate how the play 

 of external conditions accelerates or retards the reproductive 

 function. 



The influence of heat upon the reproductive powers of 

 infusorians has been carefully investigated by Maupas. The 

 higher the temperature up to a certain limit, the faster do these 

 organisms reproduce. In favourable nutritive conditions, Stylo- 

 nichia pustulata divides once in twenty-four hours at a tem- 

 perature of 7° to 10° C, twice at io° to 15°, thrice at 15° to 20°, four 

 times at 20° to 24, and five times at 24° to 27° C. Illustrating the 

 rapid rate of increase, Maupas notes 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, favouring asexual 

 reproduction and parthenogenesis rather than the sexual pro- 

 cess ; 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 favour anabolism, in others 

 katabolism. It is intelligible enough to find increased heat 

 sometimes associated with increased asexual reproduction, 

 sometimes 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 environmental influence, that of food. In another 

 ciliated infusorian {Leiicophrys), so long as food is abundant, 

 fission obtains ; but when food grows scanty, there is a metamor- 

 phosis without encystation, followed by six successive divisions. 

 These are effected, however, "without vegetative growth, and 

 have for their final object not multiplication but conjugation." 



