Echinoderm Hybridization. 149 



delicacy of the balance between an exhibition of Hipponoe characters 

 and the appearance of Toxopneustes characters. We are not surprised 

 that this should be so, now that we have seen it. It is what we might 

 expect in forms which are so closely similar. 



In comparing the change of dominance which I obtained by change 

 in alkalinity of the sea -water with that which Herbst obtained by chang- 

 ing temperature, it is interesting to notice that the normal temperature 

 of the sea-water in the tropical and subtropical regions in which my work 

 has been done is higher than those of Herbst's experimental conditions. 



In this comparison (see tables on pages 128 and 147) it will be seen 

 that the changes caused by increased temperature and the changes 

 caused by decreased alkalinity are approximately the same. I have 

 reduced the amount of Hipponoe dominance in about the same amount 

 that Herbst has decreased the amount of Strongylocentrotus influence. 

 In the cross Sphasr echinus 9 X Strongylocentrotus 6^ , Herbst has held the 

 embryo to the egg type. 



The difference between Herbst's observations and vay own is more 

 apparent than real. By one change in environment I have obtained 

 embryos of a Hipponoe type; by another change of environment, I have 

 obtained embryos of the Toxopneustes type. Herbst by combining arti- 

 ficial parthenogenesis and fertilization held the embryo to the egg type. 



As to the explanation of this phenomenon, I believe that we have 

 it in the optimum which was produced by the change in environment. 

 In work with the pure forms Herbst, Vernon, and I have shown that 

 uniformly the best results are obtained in certain definite conditions. 

 I have shown that a higher degree of alkalinity is more favorable to 

 Hipponoe, a lesser degree to Toxopneustes. If the fertilization be made 

 in the environment most favorable to the sperm, and therefore possibly 

 most favorable to the growth of its nuclear material, resemblance to 

 the male parent has followed in both cases. 



That the results are not uniform depends upon individual differences 

 in the germ-cells, although in general the germ-cells in a given parent 

 produce embryos of a similar character. This idea of individual differences 

 is simpl}^ the idea of chemical variation in the germ-cells which has arisen 

 during their growth, a variation due to food, functional reactions, etc. 



I have done nothing which will explain the "cause" of these phe- 

 nomena . I have simply shown that, in the material with which I worked, 

 definite phenomena are exhibited in certain conditions. I believe that 

 Herbst (1907) and I worked with essentially the same factors, although 

 we applied them in a somewhat different manner — in his case the method 

 of treatment being sufficient to cause parthenogenesis, in my work the 

 method of application not being sufficient. 



We have by no means exhausted all of the influential factors. Herbst 

 found a partial control in temperature. From my investigations I 

 believe that another partial control lies in the concentration of OH ions 

 in the sea-water in which development is taking place. Given an opti- 

 mum concentration, we may expect certain occurrences. 



The "unknown factor" is a complex of conditions made up of 

 factors, each one of which represents the optimum environment, an 



