156 THE LAW OF BIRTHS AND DEATHS 



during childhood is not of great importance to the 

 theory. 



But if the degree of potential fertility rises as the 

 intensity of the nervous charge falls, how is it that the 

 most fertile period is between fifteen and thirty, instead 

 of later in life when vitality begins to wane ? Take 

 the case of a highly fed, intellectual, and infertile member 

 of the wealthy classes. If a high development of nervous 

 energy leads to infertility during what should be the most 

 fertile period, why is not this infertility overcome in later 

 life by the decline of nervous energy due to advancing 

 years ? Perhaps unicellular organisms will throw some 

 light on the matter. Hertwig points out that " if nourish- 

 ment be continually and abundantly supplied to Ciliata, 

 pairing can be prevented (Maupas, vol. ii. p. 30). They 

 continue to divide until the whole culture dies off in 

 consequence of senile degeneration." * The cases are singu- 

 larly parallel. Yet, after all, this parallelism might be 

 reasonably expected on the evolutional theory. It 

 is not wise to attach too much importance to analogies, 

 or too much weight to the absence of analogies, since 

 Nature does not hesitate so to combine her principles 

 in adaptation to special circumstances that they fre- 

 quently become almost unrecognisable. But an analogy 

 so precise where we might reasonably expect an analogy 

 cannot be held as of no significance. 



First of all, we see that these unicellular organisms 

 can be prevented from conjugating by extremely favour- 

 able nutritive conditions. We should have expected 

 that the advance of senile decay would have overcome 

 the effects of favourable conditions and rendered con- 

 jugation possible. But this is not the case. The multi- 

 cellular organisms appear to react in an exactly analogous 

 1 The Cell, Dr. O. Hertwig. 



