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36 that in the vegetable kingdom as well as in the animal kingdom (I shall return later to this point) fertilisation has the best results when it occurs between parents with maximum sexual affinity.

Consideration of sexual aversion affords the readiest proof that the law holds good throughout the animal kingdom. I should like to suggest here that it would be extremely interesting to make observations as to whether the larger, heavier and less active egg-cells exert a special attraction on the smaller and more active spermatozoa, whilst those egg-cells with less food-yolk attract more strongly the larger and less active spermatozoa. It may be the case, as L. Weill has already suggested in a speculation as to the factors that determine sex, that there is a correlation between the rates of motion or kinetic energies of conjugating sexual cells. It has not yet been determined, although indeed it would be difficult to determine, if the sexual cells, apart from the streams and eddies of their fluid medium, approach each other with equal velocities or sometimes display special activity. There is a wide field for investigation here.

As I have repeatedly remarked, my law is not the only law of sexual affinity, otherwise, no doubt, it would have been discovered long ago. Just because so many other actors are bound up with it, because another, perhaps many other laws sometimes overshadow it, cases of undisturbed action of sexual affinity are rare. As the necessary investigations have not yet been finished, I will not speak at length of such laws, but rather by way of illustration I shall refer to a few factors which as yet cannot be demonstrated mathematically.

I shall begin with some phenomena which are pretty