100 ON THE ADVANTAGE Chap. IV. 



to flower on the same tree, and that flowers on the same 

 tree can be considered as distinct individuals only in a 

 limited sense. I believe this objection to be valid, but 

 that nature has largely provided against it by giving 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 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 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 have made these few remarks on 

 the sexes of trees simply to call attention to the subject. 

 Turning for a veiy brief space to animals : on the 

 land there are some hermaphrodites, as land-mollusca 

 and earth-worms ; but these all pair. As yet I have 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 view of an occasional cross being indispensable, 

 by considering the medium in winch 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 

 currents in the water offer an obvious means for an occa- 

 sional cross. And, as in the case of flowers, I have as yet 



