IV. On natural hybrids of Echinoderms. 
For a long time the Echinoderms, and especially the Echinoids, 
have been the favourite objects of artificial hybridisation. In many 
cases the larvæ have been reared to the pluteus-stage, but only in 
few cases have the hybrid larvæ been reared as far as metamorphosis 
and never to fullgrown animals. L. Doncaster!) has reared 
artificial hybrids of Paracentrotus lividus and Parechinus micro- 
tuberculatus up to metamorphosis, and in a recently published paper: 
,0On the artificial culture of marine Plankton Organisms" by E. I. 
Allen and E. W.Nelson?) it is recorded that Mr. W. De Morgan 
has succeeded in rearing artificial hybrids of Echinus esculentus 
and acutus until metamorphosis; ,,in all thirty young hybrids were 
obtained and a number of them are still alive and feeding on red 
weeds". — Herewith a most important step forwards has been 
made; the next great step will be to rear the hybrids until ma- 
turity. — Hitherto it is, however, only the hybrid larvæ which 
have been studied. But these do not give any contribution to the 
solution of several important problems relating to hybrids, e. g. the 
inheritance in the hybrids of the characters peculiar to the grown 
individuals of the parent species, or whether they become sexually 
ripe or not etc. This, of course, confines the value of such ex- 
periments within much narrower limits than would be the case, if 
the hybrids were reared to fullgrown animals. It is certainly also 
a fact of rather great importance that the fullgrown animals of 
the parent species afford several more characters, and generally 
much more important and definite, than do the corresponding larvæ. 
This will make it much more easy to follow the fate of the diffe- 
rent characters of the two parent species in the hybrids, than is 
the Case, when only the hybrid larvæ are studied. With the 
1) L. Doncaster: On rearing the later stages of Echinoid larvæ. Proc. 
Cambridge Philos. Soc. XII. 1902, p. 47—49. 
”) Quart. Journ. Mier, Se, N.S. Vol, 55. 1910, 
