286 NATURAL HISTORY. 



viduals composing any species, and to destroy all unfav- 

 ourable variations amongst the same. 



To use once more the imaginary illustration above 

 employed, the longer necks which enabled certain indi- 

 vidual giraffes to survive a drought, would be handed 

 down by inheritance to their young. On the other hand, 

 the comparatively short-necked individuals would not have 

 the chance of leaving offspring because, by the hypothesis, 

 they would be killed off. 



Moreover, in the course of this transmission, the 

 favourable variation (whatever it may be) will tend to 

 become intensified in each succeeding generation, so long 

 as the conditions which render the variation favourable 

 to the life of the individual remain in existence. So long 

 as this continues, the same process of 'selection' will go on 

 in each succeeding generation ; and the varying character 

 will become in each generation successively stronger and 

 stronger. Thus, in our illustration, so long as the 

 region tenanted by the giraffe continued subject to 

 periodic droughts, and so long as it was, therefore, good 

 for the individual giraffe to have a long neck, the 

 individuals in each generation which had the longest 

 necks would have the best chance of survival. The 

 best chance of survival, however, implies the best chance 

 of leaving offspring, and in this way the neck of the 

 giraffe might go on getting in each generation longer 

 and longer, by the preservation of the individuals which 

 possessed this variation to the greatest extent, and the 

 elimination of those with shorter necks. 



By means of this process of ' natural selection,' it is 

 easy to comprehend how 'varieties' might be produced. 



