306 WILLIAM LAWRENCE TOWER. 



if not the sole cause of the changes observed by Vernon; and 

 Herbst's work in 1906-07 on the echinoderms showed definitely 

 that temperature is a contributing, but not the exclusive fac- 

 tor. Tennent in 1910, by crossing Hipponoe esculenta with 

 Toxopneustes variegatus at Dry Tortugas, Florida, reached the 

 general result that in normal sea water Hipponoe is the 

 dominant member of the cross in the plutei; and in sea water 

 to which sodium hydrate had . been added, increasing the 

 alkalinity, he found that Hipponoe was also dominant irre- 

 spective of the direction of the cross; but when a small 

 proportion of acetic or hydrochloric axid was added to normal 

 sea water, thus decreasing the alkalinity, he found that Toxop- 

 neustes was dominant. This he considers an explanation of the 

 seasonal variation described by Vernon, but it is not quite clear 

 just what could be the basis of a seasonal variation of the alka- 

 linity or acidity of a large body of ocean water like that of the 

 Gulf of Mexico; rather, it is entirely conceivable, as Vernon 

 states and as Herbst demonstrates, that temperature is at least 

 a factor capable of producing variability in the dominance of 

 various crosses. These results of Vernon, Doncaster, Herbst and 

 Tennent deal with the early stages, and none have been carried 

 to F^ generation adults, or into the F^ and F3 generations. In 

 my crosses exactly similar results have been produced by sub- 

 jecting the organism to different sets of conditions during the 

 fertilization period, and this has brought about divergence in 

 behavior, which divergence has continued into the second and 

 third generations, and the products of the variability are in every 

 way constant and fixed from the start. 



These results indicate that in the fertilization process the 

 two somewhat unlike germinal substances that are being com- 

 bined, and interact one upon the other in exactly the same 

 way that two non-living substances would; that is, the pro- 

 ducts of the interaction are the resultant of the natures of 

 the two substances and the conditions under which the com- 

 bination took place. We must not, therefore, expect to find a 

 factor which determines dominance, such as temperature, al- 

 kalinity, moisture, etc., but rather must determine the com- 

 plex under which, when two materials are combined, definite 



