EXTERNAL INFLUENCES. 137 



land never increased so rapidly as at the 

 time of the great famine, and similar experi- 

 ence attends the periodical famines in India. 

 By the same natural law the birth-rate in 

 London is much higher in the poorest 

 parishes than in the wealthiest. 



The last efforts of life seem to be 

 devoted to the perpetuation of the race 

 rather than to the preservation of the 

 individual life. From this we may infer 

 Nature's reluctance to lose a type. 



These phenomena, whether they arise from 

 external or internal conditions, give no 

 indication of a tendency towards specific 

 variation. 



Environment. 



Darwin calls attention to the correlation 

 between the colour of animals and that of 

 the country they inhabit, and to the changes 

 in the colour of some animals during certain 

 seasons. This correlation he attributes to 

 natural selection and the struggle for exist- 

 ence. 



There is, undoubtedly, frequently a corre- 

 spondence between the colour of animals and 

 that of their habitat, and some insects 

 exactly resemble the leaves or twigs of the 



