206 CERTAIN LAWS OF VARIATION. 



variability of all characteristics may similarly diminish. 

 Should this contention be justified by subsequent re- 

 search, as seems very probable, then it might be formu- 

 lated as a definite law. This could be worded as fol- 

 lows: "The variability of a developing organism 

 diminishes regularly with its growth." Doubtless 

 there are many exceptions or partial exceptions to this 

 law. For instance, Bowditch * has shown that in the 

 case of human stature, there is a distinct increase of 

 variability just before, and at the time of, puberty in 

 boys, and a slight one in girls, followed by a decrease. 

 Weldon f has found that the variability of the frontal 

 breadths of young crabs somewhat increases with 

 growth, and then diminishes. Still these exceptions 

 are not sufficient to upset the general validity of the 

 law. As additional evidence in favour of it, may be 

 cited the observations of Bumpus $ on the variability of 

 the periwinkle. Calculating from measurements made 

 on 13,000 shells from different sources, it is found that 

 the variability in the ventricosity or relative breadth of 

 the small and medium-sized shells is distinctly greater 

 than that of the larger (and therefore on an average 

 older) shells. Again, I myself have found that the 

 variability in the size of Strongylocentrotus larvae di- 

 minishes steadily from the fifth day of growth on- 

 wards^ 



If our contentions as to rate of growth and variability 

 be admitted, it follows that the variability of embryonic 



* Report of Massachusetts State Board of Health, p. 275, 1877. 



fProc. Roy. Soc., Ivii. p. 367. 



j Zool. Bulletin, vol. i. p. 247. 



Phil. Trans. 1895, B. p. 617 (and confirmed by subsequent ob- 

 servations). 



