f>iM 



THE AMEBIC AN NATURALIST [Vol. XLIV 



6. When growth which we detect by taking into con- 

 sideration the time-factor is compared with the environ- 

 mental factors that depend upon place, the former ap- 

 parently plays the greater role in causing variations. 



7. As Urosalpinx grows larger (older) the ratio of its 

 greatest shell-aperture to its height diminishes with 

 regularity and its standard deviation tends to become 

 somewhat less. 



8. Shells from Staten Island whence the introduced 

 California shells were originally derived show greater 

 variability than the California shells. 



9. When the August shells of successive years from 

 the same localities are compared the A. M. of the ratio 

 between the greater shell-aperture and shell-height 

 fluctuates with noticeable regularity, reaching a maxi- 

 mum in 1902. 



10. Shells from the localities more exposed to the 

 beat of the waves show greater variability than those 

 from the more protected places. 



11. When dense and sparse populations are compared 

 the dense population shows less variability. 



12. The average mean of the ratio of greater shell- 

 aperture to height of shell for 50,424 Urosalpinx shells 

 is 61.662. The standard deviation is 3.367±.0071. 



13. Conclusion. — So far as the statistical method is 

 able to reveal, it is extremely doubtful whether or not 

 Urosalpinx when introduced into a new habitat ex- 

 hibits greater variability than when in its native habi- 

 tat. The change in the variability appearing in succes- 

 sive fortnights in shells from the same locality as well 

 as in change showing itself in the August shells from the 

 same locality in successive years is marked enough to 

 indicate plainly the working of an ontogenetic variabil- 

 ity independent of environmental modification, that is, 

 a ^me-factor as distinguished from a pZace-faetor. In 

 consequence of this it is practically impossible to collect 

 homologous lots of individuals of these shells upon 

 which the place- (or environmental-) factor may be ac- 

 curately determined. 



