15G LIFE AND DEATH. 



But the lengthening of the reproductive period alone may 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 in- 

 crease 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 images, 

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

 higher for the larvae and eggs, the more advantageous will it be, 

 ceteris jmribus, 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 lengthen- 

 ing 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 deposition 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 correspondingly long imago-life. The 

 Ephemeridae and Lepidoptera afford many examples of this, and in an 

 earlier work I have collected some of them l . The humming-bird 

 hawk-moth flies about for weeks laying an egg here and there, and, 

 like the allied poplar hawk-moth and lime haw T k-moth, probably 

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

 the amount of nutriment at its disposal. Many other Lepidoptera, 

 such as the majority of butterflies, fly about for weeks depositing 

 their eggs, but others, such as the emperor-moths and lappet- 

 1 See Appendix to the first essay on ' The Duration of Life,' pp. 43-46. 



