Heterogeneous hybridisation 249 
of hydroxyl ions is about 10-°N at Pacific Grove, California, and 
about 10-°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 different 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° to 0° cubic centimetre N/10 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 franciscanus 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. 
