s 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 environment 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 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 hybridisations in the ocean which in the
present state of alkalinity are practically excluded.

But granted that such hybridisations were possible, would they have
influenced the character of the fauna? In other words, are the hybrids
between sea-urchin and starfish, or better still, between sea-urchin and
mollusc, capable of development, and if so, what is their character?
The first experiment made it appear doubtful whether these heterogeneous
hybrids could live. The sea-urchin eggs which were fertilised in the
laboratory by the spermatozoa of the starfish, as a rule, died earlier
than those of the pure breeds. But more recent results indicate that
this was due merely to deficiencies in the technique of the earlier
experiments. The writer has recently obtained hybrid larvae between the
sea-urchin egg and the sperm of a mollusc (Chlorostoma) which, in the
laboratory, developed as well and lived as long as the pure breeds of
the sea-urchin, and there was nothing to indicate any difference in the
vitality of the two breeds.

So far as the question of heredity is concerned, all the experiments on
heterogeneous hybridisation of the egg of the sea-urchin with the sperm
of starfish, brittle-stars, crinoids and molluscs, have led to the same
result, namely, that the larvae have purely maternal characteristics and
differ in no way from the pure breed of the form from which the egg
is taken. By way of illustration it may