ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 341 



the hybrid oflFspring, but acted merely as a stimulus to virtual parthenogenetic 

 development of the egg. This suggestion is, however, discounted by the cytological 

 evidence. 



Evidently we have here a case of true hybridization, but the peculiar type of 

 inheritance cannot properly be compared with inheritance in other organisms until 

 at least a second generation is obtained from the hybrids. 



We have already mentioned in Section 7 that in otu" hybrids the presence of 

 characters is never so complete as in the pure forms. In the cross E. acutus ^ 

 or E. esculentus $ X E. miliaris $ , when the inheritance was maternal, as in the 

 years 1909-11, the presence of posterior epaulettes and posterior pedicellarise was 

 never so well marked as in the parent forms of E. acutus and E. esculentus. In 

 addition, the appearance of these characters in the hybrids was to a certain extent 

 delayed, and their growth, after their first appearance, was slower than in the pure 

 species. This fact has also been noticed by Debaisieux (16). Lang (50) has 

 observed that in crossing snails, the rate of development of some of the characters 

 in the young hybrids is much slower than in the pure forms. In these experiments 

 Lang crossed Helix hortensis and H. nemoralis, where the parents are red and 

 yellow. In the Fa generation he got a ratio of 3 yellow : 1 red, but as they grew 

 older this changed into a ratio of 3 red : 1 yellow. Moore (64) in a recent paper 

 has drawn attention to this fact with regard to the inheritance of skeleton chai'acters 

 in Strongyloce^itrotus lyurpuratus '^ X S.franciscanus $ . In all cases he found the 

 development of characters such as spiuousness, length of arms, and body shape, was 

 much retarded in the hybrids. He suggests that, as only half the substance that 

 produces the characters in the pure forms is present in the hybrids, their development 

 will necessarily be much slower. 



In 1912, the maternal inheritance which we had found in the previous three 

 seasons changed, in those crosses in which E. miliaris was the temale parent, to 

 paternal. (See Table orr p. 308.) The effect of this was to give a dominance, in both 

 reciprocals of a cross alike, of the late larval characters of E. esculentus and E. acutus 

 over those of E. miliaris. Whereas, however, in 1909-11 the larvse mall cultures 

 showed the same inheritance, in 1912 the general rule just stated was occasionally 

 departed from, a few experiments yielding hybrids of a mixed type. 



The reason for this change in inheritance from one year to another, a change which 

 seems to be without exact parallel, we do not know, but a number of facts seem to 

 point to some altered condition of environment having influenced the germ cells of 

 E. miliaris during the period of growth and maturation. 



Artificial changes in inheritance have been described in Echinoderms by 

 DoNCASTER (21), Herbst (39), and Tennent (90), and in Insects by Tower (93). 



Vernon (95) observed a seasonable change in the inheritance in Echinoid cros.ses 

 at Naples. Don caster (21) was able to bring this change about artificially by 



