Chap. IV.] OF INDIVIDUALS, 123 



sense. 1 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, 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 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. Of aquatic animals, there are many 

 self-fertilising hermaphrodites ; but here the currents of 

 water offer an obvious means for an occasional cross. As 



