That trees belonging to all orders have their sexes more often separated than other plants, I find to be the case in this country; and at my request Dr.Hooker tabulated the trees of New Zealand, and Dr.Asa Gray those of the United States, and the result was as I anticipated.

On the other hand, Dr.Hooker informs me that the rule does not hold good in Australia: but if most of the Australian trees are dichogamous, the same result would follow as if they bore flowers with separated sexes.

I have made these few remarks on trees simply to call attention to the subject.
Turning for a brief space to animals: various terrestrial species are hermaphrodites, such as the land-mollusca and earth-worms; but these all pair.

As yet I have not found a single terrestrial animal which can fertilise itself.

This remarkable fact, which offers so strong a contrast with terrestrial plants, is intelligible on the view of an occasional cross being indispensable; for owing to the nature of the fertilising element there are no means, analogous to the action of insects and of the wind with plants, by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals.