198 The Mechanistic Conception of Life 



of star-fish was not equally effective for these hybridizations; 

 the sperm of Asterias ochracea and A. capitata gave the best 

 results, since it was possible to fertilize from 50 per cent to 

 100 per cent of the sea-urchin eggs, while the sperm of Pycno- 

 podia and Asterina fertilized only 10 or 2 per cent respectively 

 of the same eggs. 



Godlewski used the same method for the hybridization of 

 the sea-urchin eggs with the sperm of a crinoid (Antedon 

 rosacea) . Kupelwieser afterward obtained results which seemed 

 to indicate the possibility of fertilizing the eggs of Strongylo- 

 centrotus with the sperm of a mollusk (Mytilus). Recently, the 

 writer succeeded in fertilizing the eggs of Strongylocentrotus 

 franciscanus with the sperm of a mollusk — Chlorostoma. This 

 result could only be obtained in sea-water the alkalinity of 

 which had been increased (through the addition of 0.8 c.c. n/10 

 sodium hydroxide to 50 c.c. of sea- water). We thus see that 

 by increasing the alkalinity of the sea-water it is possible to 

 effect heterogeneous hybridizations which are at present impos- 

 sible in the natural environment of these animals. 



It is, however, conceivable that in former periods of the 

 earth's history such heterogeneous hybridizations were possible. 

 It is known that in solutions like sea-water the degree of alkalin- 

 ity must increase 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 hybridizations in the ocean which in the 

 present state of alkalinity are practically excluded. 



But granted that such hybridizations were possible, would 

 they have influenced the character of the faima? In other 

 words, are the hybrids between sea-urchin and star-fish, or better 

 still, between sea-urchin and moUusks, capable of development. 



