that in this case the sex is determined before the egg, concerning which the decision is made, has come into existence. This remarkable and rare arrangement is beyond doubt connected in the first place with the peculiar smallness of the males, and with the dimorphism of the eggs which is conditioned by this. We may go the length of asking why those daughters which yield male eggs are produced when the temperature of the water is high, and why those which give rise to female eggs are produced when the temperature is low:—this could hardly be a chance effect of temperature, but is much more likely to be due to an adaptation to the peculiar conditions of life in these animals. Sexual reproduction seems often to take place late among the rotifers, when the colony has almost reached the height of its development; but this will usually coincide with the occurrence of the highest temperature, so that, by the arrangement referred to, it is secured that the males appear at the most suitable time. If this interpretation is the correct one, we may understand why it happens that in this case the determination of sex is dependent on temperature, and why the higher and lower temperature produce the effects described.
NOTE VIII (p. 28).
Watasé, in a paper which appeared in 1892, 'On the Phenomena of Sex-Differentiation' (Journ. of Morphology, vol. vi. Boston, 1892), attempted, in a manner not quite clear to me, to conceive of sexuality itself as a phenomenon dependent on stimulus. With such a view I cannot agree, as will appear more plainly further on in this lecture.
NOTE IX (p. 31).
The experiments with Musca vomitoria, here described in outline, were made in the year 1884 and 1885 and have not hitherto been published, as I intended to extend them in another direction.
