140 THE LAW OF BIRTHS AND DEATHS 



less possible. The point most favourable for fertility will 

 occupy a middle position. 



" At the very threshold of sex difference we find that 

 a little active cell or spore, unable to develop of itself, 

 unites in fatigue with a larger more quiescent individual." 1 

 This provides a strong suggestion that the sperm cell is 

 as much affected in its power of fertilisation by external 

 conditions as the ovum in its receptivity, and if this is so 

 it is probable that the declining birthrate will be almost 

 as much due to the effect of the environment upon the 

 men as upon the women of the community. The sperm 

 cell will probably be governed by the same laws as the 

 ovum in this respect. It must unite with the ovum as 

 exhaustion approaches or perish. So the fact quoted is 

 of considerable significance, especially in the case of 

 these but partially specialised organisms. 



Certain observations by Maupas on the conjugation 

 and division of unicellular organisms have become 

 classical. " In November 1885 M. Maupas isolated an 

 infusorian (Stylonichia pustulata) and observed its genera- 

 tions till March 1886. By that time there had been two 

 hundred and fifteen generations produced by ordinary divi- 

 sion, and since these lowly organisms do not conjugate with 

 near relations, there had, of course, been no sexual union. 



" What was the result ? At the date referred to the 

 family was observed to have exhausted itself. The 

 members, though not exactly old, were being born old. 

 The asexual division came to a standstill, and the powers 

 of nutrition were also lost. 



" Meanwhile, however, several of the individuals before 

 the generations had exhausted themselves had been re- 

 moved to another basin, where they conjugated with 

 unrelated forms of the same species. One of these 

 was again isolated and watched for five months. The 

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



