AMONG UNICELLULAR ORGANISMS 143 



and vigour conjugation is unnecessary ; but when condi- 

 tions become adverse to the vitality of the organisms 

 and exhausting, they must either conjugate or perish. 

 Thus the parallel between the higher organisms and the 

 unicellulars is quite close. 



The principle of the vital optimum is clearly manifested 

 by these unicellular organisms. " The action of heat 

 may be two-fold : up to a certain point it quickens develop- 

 ment and the general life, favouring 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 

 favour anabolism, in others katabolism. It is intelli- 

 gible enough to find increased heat sometimes associated 

 with increased asexual reproduction, sometimes with 

 increased sexuality. Instances of both may be gathered 

 from Sempers' Animal Life, the classical work on the 

 influence of environment upon the organism." x These 

 remarks summarise the facts in regard to both multi- 

 cellular and unicellular organisms. As each type of 

 organism will be adapted to a certain range of temperature, 

 it is an obvious inference that any change in either direc- 

 tion will be " sometimes associated with increased asexual 

 reproduction, sometimes with accelerated sexuality," ac- 

 cording to whether it is stimulating or exhausting, this 

 depending on its relation to the temperature to which the 

 organisms are adapted. Another point to remember is 

 that the optimum point, whether for vitality or fertility, 

 is not absolutely fixed but variable, so that change is 

 often beneficial to the organism. 



" Finally, mature animal egg cells, even when under 

 normal conditions in the ovary or in the oviducts, live 

 1 Evolution of Sex, Geddes and Thomson, chap. xvii. 



