226 The American Naturalist. [Marech, 
The two species that I have used in my research are Echinus 
microtuberculatus and Sphzrechinus granularis. I am certain 
that amore favorable combination than this can be found, for 
although the two species are well adapted inasmuch as their 
larvee differ very considerably from one another, yet there is to 
Fig. 1. Echinus microtuberculatus larva; pure type. 
be taken into account, the fact that cross-fertilization between 
the two is so difficult that in my experiment out of 1000 picked 
eggs (Sphzrechinus 9, Echinus ¢) nota single fertilized egg 
reattiod Of the four species found at Naples in great pba 
. 
