182 NATURAL INHERITANCE. [chap. 



sufficient light. Longevity, marriage, and fertility 

 must all be affected by the amount of consumptivity, 

 whereas in the case of the faculties hitherto discussed 

 they are not affected to any sensible extent. It how- 

 ever happens that these influences tend to neutralize 

 one another. It is true that consumptive persons die 

 early, and many of them before a marriageable age. 

 On the other hand, they certainly marry earlier as a 

 rule than others, one cause of which lies in their 

 frequent great attractiveness ; and again, when they 

 marry, they produce children more quickly than others. 

 Consequently those who die even long before middle 

 age, often contrive to leave large families. The greater 

 rapidity with which the generations follow each other, 

 is also a consideration of some importance. There 

 is therefore a fair doubt whether a group of young 

 persons destined to die of consumption, contribute 

 considerably less to the future population than an 

 equally large group who are destined to die of other 

 diseases. I will at all events assume that consumptivity 

 does not affect the numbers of the adult children, 

 simply as a working hypothesis, and will afterwards 

 compare its results with observed facts. 



I should add that the question whether the sexes 

 transmit consumption equally, lies outside the present 

 work, at least for practical purposes ; for whether they 

 transmit it equally or not would not affect the results 

 materially. Our list of data is therefore limited to 

 these : — that 16 per cent, of the population die of 

 consumption, that consumptivity is normally distri- 



