160 Controversial Questions. 



already said, firmly established : comparative and experi- 

 mental methods are just coming on. 



I propose, therefore, to suggest a series of problems 

 the solution of which would in my opinion throw much 

 light on the essential difference between mutability and 

 variability. 



1. More examples of Quetelet's law are wanted: 

 their number can never become too great. 



2. The curves in question should be plotted from the 

 same individuals or from the same batch of individuals 

 in successive years. The constancy of their means and 

 their amplitude (Galton's and Q') should be deter- 

 mined. Changes in these values, and changes in the 

 symmetry of the curves, should if possible be traced to 

 their causes. 



3. Polymorphic curves should be looked for and ana- 

 lysed. These may point to the existence of mixtures of 

 perfectly distinct elementary species growing together 

 or to the existence of antagonistic characters within the 

 limits of a single species (for examples annual and bi- 

 ennial forms in Daucits, Beta, etc.) They may also be 

 due to diseases. Finally they may be the so-called ''double 

 curves" in which the several apices are to be regarded 

 as ordinates in a curve of a higher order, and not as 

 indications of mutation. 



4. Correlative variation is a phenomenon of the high- 

 est importance.^ For example man presents many ni- 

 stances of correlation between mental and physical char- 

 acters. Correlations fall in two categories. In the one 

 are those cases in which the two characters are dependent 

 in the same way although not to the same degree on ex- 



*J. H. BuRKiLL, Variation in the Number of Stamens and Car- 

 pels, Journ. Linn. Soc. Bot, Vol. 31. 



