THE STKUGGLE FOR EXISTENCE. 69 



heavier and larger than the latter ; but significant also 

 was the discovery that the range of variability was dis- 

 tinctly smaller among the survivors than the eliminated, 

 thus proving that those individuals which departed least 

 from the " ideal type " had on the whole the best chance 

 of surviving. Weldon has also shown, in the case of a 

 terrestrial mollusc, Clausilia laminata, that extreme vari- 

 ations tend to be eliminated. Selection of this kind pre- 

 serves the mean character of the race. Tower obtained 

 the same results on comparing Chrysomelid beetles 

 (Leptinotarsa decemlineata) before and after hibernation ; 

 the extreme variants died off, and the survivors ap- 

 proached nearer to the mean. 



The fact that many commonly occurring mutations, 

 such as albinism or variegate'ci leaves among plants, fail 

 to establish themselves is also clear evidence that con- 

 stant elimination of these variations must take place. 



Of all the agents of elimination among human beings 

 disease is the most potent in modern times, as Reid and 

 others have well shown. Some notion of the death-rate 

 and its causes in the human species may be gathered from 

 the official government reports. In the year 1909 there 

 died in England and Wales 518,000 individuals (in round 

 numbers), of whom 100,000 were infants under one year 

 of age. Some 18,000 persons were killed by violence or 

 accident, while the remaining 500,000 died of disease. 

 Of these tuberculosis carried off 55,000, pneumonia 

 36,000, cancer 32,000, measles 12,000, diarrhoea 10,000, 

 influenza 9000, whooping-cough 7000, diphtheria 5000, 

 scarlet fever 3000, enteric fever 2000, and other diseases 

 smaller numbers. This death-rate is selective in so far 

 as it affects the fertility and prevents the reproduction 

 of susceptible individuals. 



The germs of most of these diseases are so widespread 

 that infection cannot be avoided ; therefore a constant 

 elimination takes place of individuals unable to resist 

 their attacks. If the resistance to a particular dangerous 



