Hillside. 1 5 1 



on account of their close resemblance to the surfaces on 

 which they rested; and probably you have seen enough 

 during our rambles together to enable you to understand 

 that those individuals in whom this resemblance is the most 

 perfect will have the best chance of escaping the kind 

 attentions of the birds, and of bringing up a family. But 

 as the progeny resemble (more or less perfectly) their 

 parents, it becomes clear that in time a race will be pro- 

 duced which is exactly suited to its environment. 



Let us suppose now that such a race has come into ex- 

 istence which is adapted to the tree- trunks and fences of 

 some clean-aired country side. But a manufacturing centre 

 springs up in the locality, with the air-polluting and surface- 

 darkening consequences to which we have already alluded. 

 No longer are the darker varieties more conspicuous than 

 what has hitherto been the prevailing type ; in fact, the 

 reverse is now the case. It is these darker forms, therefore, 

 that now have the advantage in the changed environment ; 

 the old typical form becomes more conspicuous, the birds 

 pick these off, and soon only the darkest specimens have a 

 chance of escape. The darkening from increased smoke 

 goes on, the moths in each successive generation tend to 

 get blacker, and in time a perfectly black race may be 

 developed. This is no imaginary condition of things, but 

 one which has occurred during the last half-century in a 

 great number of species. 



Thus the Great Mother works on. Ever toiling, never 

 weary, she needs no rest ; but night and day, summer and 

 winter, she labours on, producing in infinite variety wonder- 

 ful and marvellous results. Not an object around us but 

 shows us the result of her handiwork, of her loving care ; 



