Heterogeneous hybridisation 249 



of hydroxyl ions is about 10~ 6 JV at Pacific Grove, California, and 

 about 10~ 5 N at Woods Hole, Massachusetts). If we slightly raise 

 the alkalinity of the sea-water by adding to it a small but definite 

 quantity of sodium hydroxide or some other alkali, the eggs of the 

 sea-urchin can be fertilised with the sperm of widely diiferent groups 

 of animals, possibly with the sperm of any marine animal which sheds 

 it into the ocean. In 1903 it was shown that if we add from about 

 0'5 to 0'8 cubic centimetre N/W sodium hydroxide to 50 cubic 

 centimetres of sea-water, the eggs of Strongylocentrotus purpuratus 

 (a sea-urchin which is found on the coast of California) can be 

 fertilised in large quantities by the sperm of various kinds of starfish, 

 brittle-stars and holothurians ; while in normal sea-water or with 

 less sodium hydroxide not a single egg of the same female could be 

 fertilised with the starfish sperm which proved effective in the 

 hyper-alkaline sea- water. The sperm of the various forms of starfish 

 was not equally effective for these hybridisations ; the sperm of 

 Asterias ochracea and A. capitata gave the best results, since it was 

 possible to fertilise 50 % or more of the sea-urchin eggs, while the 

 sperm of Pycnopodia and Asterina fertilised only 2 / of the same 

 eggs. 



Godlewski used the same method for the hybridisation of the sea- 

 urchin eggs with the sperm of a crinoid (Antedon rosacea). Kupel- 

 wieser afterwards obtained results which seemed to indicate the 

 possibility of fertilising the eggs of Strongylocentrotus with the 

 sperm of a mollusc (Mytilus). Recently, the writer succeeded in 

 fertilising the eggs of Strongylocentrotus franciscamis with the 

 sperm of a mollusc Chlorostoma. This result could only be obtained 

 in sea-water the alkalinity of which had been increased (through the 

 addition of 0*8 cubic centimetre N/10 sodium hydroxide to 50 cubic 

 centimetres of sea-water). We thus see that by increasing the 

 alkalinity of the sea- water it is possible to effect heterogeneous 

 hybridisations which are at present impossible in the natural en- 

 vironment of these animals. 



It is, however, conceivable that in former periods of the earth's 

 history such heterogeneous hybridisations were possible. It is known 

 that in solutions like sea-water the degree of alkalinity must in- 

 crease when the amount of carbon-dioxide in the atmosphere is 

 diminished. If it be true, as Arrhenius assumes, that the Ice age 

 was caused or preceded by a diminution in the amount of carbon- 

 dioxide in the air, such a diminution must also have resulted in an 

 increase of the alkalinity of the sea-water, and one result of such an 

 increase must have been to render possible heterogeneous hybridi- 

 sations in the ocean which in the present state of alkalinity are 

 practically excluded. 



