INTR OD UC TIOAT. 5 



variations as arise and are beneficial to the being under 

 its conditions of life. No one objects to agriculturists 

 speaking of the potent effects of man's selection, and 

 in this case the individual differences given by nature, 

 which man for some object selects, must of necessity 

 first occur." It is evident then that Mr. Darwin did 

 not attempt to account for the origin of variations, but 

 that the service rendered by him and by Mr. Wallace 

 to the doctrine of evolution consists in the demonstra- 

 tion of the reality of natural selection. Darwin also 

 assumes in the statement first quoted above, the in- 

 heritance of acquired characters. 



In 1865 the Principles of Biology of Herbert Spen- 

 cer appeared. In this work the attempt is made to 

 set forth the laws of organic evolution, in a way which 

 represents an advance beyond the positions of his 

 predecessors. He adopts and harmonizes both the 

 Lamarckian and Darwinian doctrines, and is at times 

 more specific in his application of Lamarck's doctrine 

 of the stimulus of the environment, and of use, than 

 was Lamarck himself. Very often, however, Spencer 

 contents himself with generalities ; or takes refuge in 

 the "instability of the homogeneous," as an efficient 

 cause. This phrase, however, like his other one, "the 

 unknowable," is but a makeshift of temporary ignor- 

 ance, and is neglected by Spencer himself, when he 

 can see his way through it. He approaches the cause 

 of the varied forms of leaves of plants in this language : l 

 "And it will also be remembered that these equalities 

 and inequalities of development correspond with the 

 equalities and inequalities in the incidence of forces." 

 Language of similar significant but rather indefinite 

 import is frequently used throughout this volume. 



1 The Principles of Biology, by Herbert Spencer, Amer. Ed., 1873, II., p. 143. 



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