ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 



325 



hybrids, future investigation alone can show. It may be asked why we should not 

 find a difference, on these grounds, between the inheritance for the seasons repre- 

 sented by 1 or 2, as compared with that of 3 ; for 3 is decidedly different from 

 1 or 2. It will be noted, however, that the difference between 3 and 4 is almost 

 twice as great as that between I or 2 as compared with 3. 



Text-fig. 17. — Temperature curves for the four seasons, during the period of the ripening of the germ 

 cells in Echimis. They are based on weekly means of the temperature of the sea-water in Plymouth 

 Sound during the years our work has been in progress. 1 = 1908-1909; 2 = 1909-1910. 

 3 = 1910-1911 : 4 = 1911-1912. 



It will be remembered that the peculiarity of inheritance in 1912 was particularly 

 connected with E. miliaris. In the region of the open sea, in 20-30 fathoms of 

 water, E. esculentus and E. acutus are exposed to little or no temperature variation, 

 as compared with E. miliaris in shallow shore water. We should not be surprised, 

 therefore, to find E. esculentus and E. acutus showing no difference from former 



