SOURCES OF CHANGE 5S 



materials received from its parents, is susceptible 

 to modification from within or from without, and 

 if so, whether the modification is of any importance 

 in the evolution of species. This matter is at the 

 heart of the problem with which this work is con- 

 cerned, and consequently involves more questions 

 than the mere possibility of change in the indi- 

 vidual. It will be considered later from several 

 points of view. 



Changes in organisms due to any of these factors 

 constitute the individual differences in a species, as 

 well as the variability of the individual through 

 the successive periods of its life. They are the 

 variations which are so universally evident among 

 living things, and regardless of their immediate 

 cause, must be looked upon as potentially of inter- 

 est in any study of evolution. 



The attitude of scientists to this problem has 

 been an interesting evidence of the degree of ex- 

 planation which may be satisfactory to scientific 

 minds. Darwin ^ recognized the occurrence of 

 fortuitous variations without making any attempt 

 to explain them on a basis of cause and effect; as 

 some writers have expressed his attitude, he re- 

 garded them as axiomatic. Nevertheless he was 

 not unaware of the effectiveness of Lamarckian 

 principles in producing variations. He noted the 



* (a) The Foundations of the Origin of Species, A Sketch Written in 18I^2, 

 edited by Francis Darwin, Cambridge, 1909; (6) The Origin of Species, 1859; 

 (c) The Descent of Man, 1871. 



