100 
ON THE ADYANTAGE 
Chap. IY. 
Table flowers, it may be objected that pollen could seldom 
be carried from tree to tree, and at most only from flower 
to flower on the same tree, and that flowers on the same 
tree can be considered as distinct indiYiduals only in a 
limited sense. I belicYe this objection to be Yalid, but 
that nature has largely proYided against it by giYing to 
trees a strong tendency to bear flowers with separated 
sexes. When the sexes are separated, although the 
male and female flowers may be produced on the same 
tree, we can see that pollen must be regularly carried 
from flower to flower; and this will give a better chance 
of pollen being occasionally carried from tree to tree. 
That trees belonging to all Orders haYe 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 anti¬ 
cipated. On the other hand. Dr. Hooker has recently 
informed me that he finds that the rule does not hold 
in Australia; and I haYe made these few remarks on 
the sexes of trees simply to call attention to the subject. 
Turning for a Yery brief space to animals: on the 
land there are some hermaphrodites, as land-mollusca 
and earth-worms ; but these all pair. As yet I haYe not 
found a single case of a terrestrial animal which fer¬ 
tilises itself. We can understand this remarkable fact, 
which offers so strong a contrast with terrestrial plants, 
on the Yiew of an occasional cross being indispensable, 
by considering the medium in which terrestrial animals 
live, and the nature of the fertilising element; for we 
know of no means, analogous to the action of insects and 
of the wind in the case of plants, by which an occasional 
cross could be effected with terrestrial animals without 
the concurrence of two individuals. Of aquatic animals, 
there are many self-fertilising hermaphrodites; but here 
