158 LIFE AND DEATH. [III. 



result in a marked increase in the length of life, as is proved 

 by the queen-bee. In all these cases it is easy to imagine the 

 operation of natural selection in producing such alterations in 

 the duration of life, and indeed we might accurately calculate 

 the amount of increase which would be produced in any given 

 case if the necessary data were available, viz. the physiological 

 strength of the body, and its relations to the external world, 

 such as, for instance, the power of obtaining food at various 

 periods of life, the expenditure of energy necessary for this 

 end, and the statistics of destruction, that is, the probabilities 

 in favour of the accidental death of a single individual at any 

 given time. These statistics must be known both for the 

 imagos, larvae and eggs ; for the lower they are for the 

 imagos, and the higher for the larvae and eggs, the more 

 advantageous will it be, ceteris paribus, for the number of eggs 

 produced by the imago to be increased, and the more probable 

 it would therefore be that a long reproductive period, involving 

 a lengthening of the life of the imago, would be introduced. 

 But we are still far from being able to apply mathematics to 

 the phenomena of life ; the factors are too numerous, and 

 no attempt has been made as yet to determine them with 

 accuracy. 



But we must at least admit the principle that both the 

 lengthening and shortening of life are possible by means of 

 natural selection, and that this process is alone able to render 

 intelligible the exact adaptation of the length of life to the 

 conditions of existence. 



A shortening of the normal duration of life is also possible ; 

 this is shown in every case of sudden death, after the deposi- 

 tion of the whole of the eggs at a single time. This occurs 

 among certain insects, while nearly allied forms of which the 

 oviposition lasts over many days therefore possess a corre- 

 spondingly long imago-life. The Ephemeridae and Lepidoptera 

 afford many examples of this, and in an earlier work 1 have 

 collected some of them ^ The humming-bird hawk-moth flies 

 about for weeks laying an egg here and there, and, like the 

 allied poplar hawk-moth and lime hawk-moth, probably dies 

 when it has deposited all the eggs which can be matured with 



' See Appendix to the first essay on * The Duration of Life,' pp. 40, 

 43 46. 



