HEREDITY AND EVOLUTION 85 



acquisitions of parental life. Individual life receives 

 and transmits again. While, however, variations are 

 possible, sudden changes in the structure of organism 

 do not appear. Natural selection acts only by the 

 accumulation of slight modifications, . . . each profit 

 able to the individual under its conditions of life. 1 

 The work of the naturalist has thus two sides, the 

 one to_trace the long and graduated succession of 

 modifications ; the other to discover the laws of 

 heredity in accordance with which slight modifica 

 tions may be transmitted. Variation is one side, trans 

 mission only the other side of the same movement. 

 Modified structure will impress some corresponding 

 modification on the structures and polarities of its 

 units. 2 The units and the aggregates must act and 

 react on each other. 3 



A breadth of fresh light has been thrown over the 

 difficult problem of heredity, by extended research into 

 the minute variations that appear in the history of 

 species, and of families belonging to the same species. 

 By concentration on detailed illustrations it has 

 become easier to trace the lines of reproduction. It 

 has proved possible to mark the rise of slight variations 

 and to trace their continuance in the life of progeny. 

 That there are fixed laws of inheritance has thus 

 become matter of certainty. It seems natural to 

 suppose that favourable variations in structure would 

 tell upon the reproductive elements. But a theory 

 of heredity proves as difficult to make out as a theory 

 of acquisition. Indeed, the difficulty as to heredity 

 seems even greater, inasmuch as the action of 



1 Origin of Species, p. 211. 



2 Spencer s Principles of Biology, i. p. 256. 3 Ibid. 



