41 



c. Unfavorable climate. 



The winter of 1854-55 in England. 



Recapitulation. 



The Complexity of the Struggle. 



Hypothetical example. 



Cats and the crop of clover seed. 



Salmon and the inland birds. 



Trees on the heath in Staffordshire. 

 Birth rate proportional to the risk of destruction. 



Contrast fishes and petrel. 



The passenger pigeon. 



iii. NaUiral Selection. 



(Darwin, Origin of Species, Chap. IV ; Wallace, Darwinism, 

 pp. 102-151, 187-267, and 301-337; Lloyd Morgan, Aninial 

 Life and Intelligence, pp. 77-121 ; Romanes, Darwin and after 

 Darwin, pp. 251-378 ; Marshall, Lectures on the Darwinian 

 Theory, pp. 27-52 and 116-172.) 



a. Evidences for the theory. 



(i) The observed fact that the struggle for ex- 

 istence leads to the extermination of forms 

 less fitted for the struggle and thus makes 

 room for forms more fitted. 



(2) Not a single structure or instinct in the ani- 



mal or vegetable kingdom is developed 

 for the exclusive benefit of another spe- 

 cies. 



Apparent objections : — 



Secretion of aphides useful to ants. 



Vegetable galls of use to insects. 



(3) The efficacy of artificial selection. 



(See Romanes, figs. 91-107 ; Marshall, figs. 1-3 ; Bailey, 

 Plant-Breeding.) 



