Some Subjects for Future Investigation. 161 



ternal conditions. In the other are those cases in which 

 variation in one character is the cause of variation in 

 another/ as for example the various phenomena of 

 growth which are correlated with differences in photo- 

 synthetic activity. It is superfluous to refer the reader 

 to Galton's method of studying correlation.^ 



5. The relation between external conditions of life 

 and variability ought to be investigated. Are there vari- 

 ations which are independent of such, or are there not? 

 If there are, what are their causes? Do the individual 

 external factors exert a separate influence or not? Is 

 there a definite relation between the extent of this in- 

 fluence and the magnitude of the variation? Do all 

 characters under the influence of high nutrition vary in a 

 plus direction, and under a poor one in a minus direc- 

 tion ?^ 



6. The sensitive period in the development of char- 

 acters should be determined. When the rudiments of 

 organs are visible under the microscope it is usually too 

 late to exert any restraining influence on their develop- 

 ment. But there may be exceptions to this rule. During 

 the time which a character takes to develop there is prob- 

 ably one short period of extreme susceptibility; and this 

 may be gradually attained and gradually lost. Here is 

 matter for much interesting inquiry. 



7. Galton^s regression is very important. Suppose 

 we sow seeds of a self-fertilizing plant: and suppose 

 that we know the amount by which it deviates from the 



^DuNCKER, Roux's Avchiv, Vol. VIII, p. 163. 



^ Galton, Natural Inheritance, and Proceedings Royal Society, 

 Vols. 40 and 45 ; and Ed. Verschaffelt, Correlaticve Variatie by 

 planten. Botan. Jaarboek, VIII, p. 92. 



^Variability can also he influenced by grrafting and inoculation. 

 See L. Daniel, Compt. Rend., 1894, T. CXVIII, p. 992. 



