192 THE CONTROL OF LIFE 



— in short, all manner of shifts and devices. They 

 use their fitnesses and they frequently strike out little 

 improvements in these. This answering back to the 

 environing difficulties and limitations constitutes the 

 struggle for existence, especially when there is some- 

 thing novel and individual in the answer. The strug- 

 gle for existence is a technical and metaphorical term ; 

 it need not be sanguinary ; it sometimes leads to the 

 sifting out of the relatively fitter to given conditions. 

 The great law of life is that species survive in virtue 

 of quahties of relative fitness. Fittest does not mean 

 strongest, cleverest, gentlest — it means best adapted 

 to the given conditions. The tapeworm is as fit for 

 its inglorious fife of ease in the food canal of a dog 

 as the lark at heaven's gate. Some animals survive 

 because they are strong, others because they are clever, 

 others because they have found a safe habitat (such 

 as a cave), others because they are so quick, others 

 because they have a garment of invisibihty. But 

 all succeed because they have certain quahties of fit- 

 ness. Professor Punnett calculates that if in a popu- 

 lation of 10,000 wild animals in a district there were 

 10 of a new and promising variety, which had a 5 per 

 cent, selection advantage over the original forms, the 

 latter would almost completely disappear in less than 

 a hundred generations, and of course there might be 

 several generations, as in rats and rabbits, in a year. 

 So if the sifting is consistent and persistent a change 

 may come about in a comparatively short time. A 

 dark variety of the Pepper Moth (Amphidasys betu- 



