NATURAL SELECTION 79 



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 occasion- 

 ally 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 tabu- 

 lated 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 at- 

 tention 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 ani- 

 mal which can fertilize 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 fertilizing element there are no means, analogous to the ac- 

 tion of insects and of the wind with plants, by which an occasional 

 cross could be effected with terrestrial animals without the con- 

 currence 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 consultation with one of the 

 highest authorities, viz., Professor Huxley, to discover a single 

 hermaphrodite animal with the organs of reproduction so perfectly 

 enclosed 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, under this point of 

 view, a case of great difficulty; but I have been enabled, by a fortu- 

 nate chance, to prove that two individuals, though both of self- 

 fertilizing hermaphrodites, do sometimes cross. 



It must have struck most naturalists as a strange anomaly that, 

 both with animals and plants, some species of the same family and 

 even of the same genus, though agreeing closely with each other in 

 their whole organization, are hermaphrodites, and some unisexual. 

 But if, in fact, all hermaphrodites do occasionally intercross, the 

 difference between them and unisexual species is, as far as func- 

 tion is concerned, very small. 



