ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 343 



that in not a single instance out of oiu' numerous cultures during 1909-11 did the 

 type of inheritance show any change. If the inheritance in our larvae were of the 

 nature of that described by Tennent for Hipponoii and Toxopneustes, that is that it 

 can be influenced by some external condition, such as the reaction, salinity, and 

 temperature of the sea-water at the time of, or after fertilization, then it is remarkable 

 that in all these cultures, in many of which these conditions must have varied widely, 

 no stimulus should have been present capable of bringing about some change. 



The fact that no such change took place in any of these cultures lends a certain 

 amount of support to our belief that, whatever may have been the cause that brought 

 about the change of inheritance in 1912, it was one that acted on the germ cells 

 during their period of growth and maturation, and not at the time of fertilization or 

 afterwards. 



We suggest that in the variation of the water temperature in the winter of 

 1911-12 is possibly to he sought this disturbing factor ; our records, however, do not 

 extend over a sufficient number of years to allow us to form at present a definite 

 conclusion. It will remain for future investigation to prove or disprove this 

 suggestion. 



It is certain that something adversely affected the eggs of JE. iniliaris in 1912 so 

 that these were unable to transmit their characters to the hybrid offspring as in 

 previous years. 



We have clearly established that the type of inheritance may change from year to 

 year in the same cross. This has been shown with regard to perfectly definite and 

 invariable characters of the late larval life. 



This affords a certain amount of confirmation of Tennent's results, based on the 

 more variable characters of the early larval skeleton. Tennent, however, was 

 dealing with a different type of inheritance. Our own is not strictly comparable 

 with his, as we have explained in Section 12. We both agree that, whatever 

 the type of inheritance, it can change from time to time, wliatever this may signify. 



Besides the cases referred to above of artificially produced changes in inheritance, 

 there are one or two cases in which no causes are known to explain the observed 

 alterations. 



Kbllog (46) described what he called "individual idiosyncrasy" in the behaviour 

 of certain silk-worms when crossed with one another. 



In one experiment, Italian silk- worms (with salmon cocoon) $ X Bagdad (white 

 cocoon) $ gave Fi all with salmon cocoons. 



In another experiment all the Fj individuals formed white cocoons. This may be 

 compared with the fact that in 1912 most of the experiments showed a dominance of 

 E. esculentus or E. acutus characters over E. miliaris ; in a few cases the inheritance 

 was otherwise. 



Arnold Lang (51) crossed the closely related species of snails Tachea hortensis 

 and T. nemoralis. Sometimes he got hybrids which followed the usual Mendelian 



