THE 



AMERICAN NATURALIST 



Vol. XLI October, 1907 No. 490 



STUDIES OF GASTROPODA 



III. Ox Ortitogexetic Varl\tiox IX Gastropoda^ 



AMAI)i;rs \V. CliAIiAU^ 



Oktdogkxetic variation niav l)e delined as prof^ressive variation 

 along definite or determinate lines, whether such variation is along 

 the line of increasing or decTeasing complexity; i. e., aggradation al 

 or degradational. In the first place orthogenetic variation is 

 ontogenetic; i. e., the successive changes which the individual 

 undergoes in its transformation from embryo to adult follow each 

 other in definite succession, the cliaiiges appearing step l)y step. 

 When w^e are conviuccnl that the changes seen in the development 

 of the individual are reniinisccnt of tlic changes passed through bv 

 its successive ancct-.r., it hec.unc. appaivnt that pln/l n/mctic 

 variation is also orthogmrtic ..r ah-ng .h-finitcly .Ictcnninahlc hues. 



ibleness of the changes whicli the individual undergoes during its 

 embryonic period, to the adults of more ])riniitive types. Usuallv 

 the comparison has been with adults of existing types, since in 

 most cases these alone were available for com])aris()n. It is no 



