OP mMVID tTALS. 93 



tree to tree. That trees belonging to all orders have their 

 sexes more often separated than other plants, I find to be 

 tho case in this country; and at my request Dr. Hooker 

 tabulated the trees ©f New Zealand, and Dr. Asa Gray, 

 those of the Uhited States, and the result was as I antici- 

 pated. 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 f jllow 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 fertilize itself. This 

 remarkable fact, which offers so strong a contrast with ter- 

 restrial plants, is intelligible on the view of an occasional 

 cross being indispensible; 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 consultation with one of the highest 

 authorities, namely. Professor Huxley, to discover a single 

 hermaphrodite animal with the organs of reproduction so 

 perfectly enclosed that access from without, and the occa- 

 sional 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 difH- 

 culty; but I have been enabled, by a fortunate 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 

 hermaphrodities, and some unisexual. But if, in fact, all 

 hermaphrodities do occasionally intercross, the difference 

 between them and unisexual species is, as far as function 

 is concerned, very small. 



