NATURAL SELECTION 147 



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 dichog- 

 amous, the same result would follow as if they bore 

 flowers with separated sexes. I have made these few re- 

 marks on trees simply to call attention to the subject. 



Turnmg for a brief space to animals: various terres- 

 trial 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 fertilize itself. 

 This remarkable fact, which ofiers 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 fertilizing 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- 

 fertilizing hermaphrodites; but here the currents of water 

 offer an obvious means for an occasional cross. As in 

 the case of flowers, I have as yet failed, after consulta- 

 tion with, one of the highest authorities, namely. Professor 

 Huxley, to discover a single hermaphrodite animal with 

 the organs of reproduction so perfectly inclosed that ac- 

 cess from without, and the occasional influence of a 

 distinct individual, can be shown to be physically 

 impossible. Cirripeds long appeared to me to present, 

 under this point of view, a case of great difficulty; but 

 I have been enabled, by a fortunate chance, to prove 



