NATURAL SELECTION 91 



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 anticipated. 

 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 very 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 

 fertilises 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 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 currents in 

 the water offer an obvious means for an occasional 

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

 failed, after consultation with one of the highest 

 authorities, namely, Professor Huxley, to discover a 

 single case of an hermaphodrite animal with the organs 

 of reproduction so perfectly enclosed within the body, 

 that access from without and the occasional influence 

 of a distinct individual can be shown to be physically 

 impossible. Cirripedes long appeared to me to present 

 a case of very great difficulty under this point of view ; 



