CHAP. XXI. SPECIES BY NATURAL SELECTION. 409 



Varieties by natural Means of Selection/' to appear at the 

 same time.* 



B}^ reference to these memoirs it will be seen that both 

 writers begin by applying to the animal and vegetable worlds 

 the Malthusian doctrine of population, or its tendency to in- 

 crease in a geometrical ratio, while food can only be made to 

 augment even locally in an arithmetical one. There being, 

 therefore, no room or means of subsistence for a lai'ge pro- 

 portion of the plants and animals which are born into the 

 world, a great number must annually perish. Hence there 

 is a constant struggle for existence among the individuals 

 which represent each species, and the vast majority can 

 never reach the adult state, to say nothing of the multitudes 

 of ova and seeds which are never hatched or allowed to 

 germinate. Of birds it is estimated that the number of those 

 which die every year equals the aggregate number by which 

 the species to which they respectively belong is on the ave- 

 rage permanently represented. 



The trial of strength which must decide what individuals 

 are to survive and what to succumb, occurs in the season 

 when the means of subsistence are fewest, or enemies most 

 numerous, or when the individuals are enfeebled by climate 

 or other causes; and it is then that those varieties which 

 have any, even the slightest, advantage over others come off 

 victorious. They may often owe their safety to what would 

 seem to a casual obsei-ver a trifling difference, such as a darker 

 or lighter shade of color rendering them less visible to a 

 species which preys upon them, or sometimes to attributes 

 more obviously advantageous, such as greater cunning, or 

 superior powers of flight or swiftness of foot. These peculiar 

 qualities and faculties, bodil}^ and instinctive, may enable them 

 to outlive their less favored i-ivals, and, being transmitted 



* See Proceedings of Linnsean Society, 1858. 



