AMONG UNICELLULAR ORGANISMS 189 



which sexual reproduction is the rule, have to meet two 

 opposite sets of conditions. There is the case of an 

 improving environment with a declining deathrate, when, 

 if evolution is not to be self-defeating, the birthrate 

 must diminish also. Conversely, when the deathrate 

 increases the birthrate must increase also, or the race 

 will become extinct. But there are conditions of excep- 

 tional hardship (usually temporary), such as famine, 

 when an increased birthrate, instead of compensating 

 for an increased deathrate, would merely add to the 

 mischief. Therefore we may expect to find, and do 

 find, that these organisms are rendered sterile both by 

 excessively favourable and excessively unfavourable con- 

 ditions. The same holds good of plants which are rendered 

 sexually sterile both by extremely favourable and extremely 

 unfavourable environments. So that the condition most 

 favourable to fertility, most favourable, that is, to the 

 union of sperm cell and ovum, occupies a middle position. 



This point most favourable to fertility may be termed 

 the optimum point for fertility. It is probable that 

 from this point the capacity for fertilisation decreases 

 steadily with the increase of vitality, but that when the 

 degree of vitality is lowered beyond the optimum point 

 for fertility the capacity for fertilisation steadily decreases 

 with the decrease of vitality. It is, indeed, difficult to 

 see what other arrangement is possible. 



The usual method of reproduction among unicellular 

 organisms is by division, conjugation taking place periodic- 

 ally. Therefore reasoning by analogy with what we 

 have seen to be the rule among the higher organisms, 

 we should expect that there will be an optimum point 

 for conjugation that conjugation will become more and 

 more imperative with these lowly organisms as exhaus- 

 tion approaches ; but that when exhaustion is carried 

 beyond a certain point conjugation will become gradually 



