CHAP. IV.] OF INDIVIDUALS. 73 



were true to their kind, and some even of these were not perfectly 

 true. Yet the pistil of each cabbage-flower is surrounded not 

 only by its own six stamens but by those of the many other 

 flowers on the same plant ; and the pollen of each flower readily 

 gets on its own stigma without insect agency ; for I have found 

 that plants carefully protected from insects produce the full 

 number of pods. How, then, comes it that such a vast number of 

 the seedlings are mongrelized ? It must arise from the pollen 

 of a distinct variety having a prepotent effect over the flower's 

 own pollen ; and that this is part of the general law of good being 

 derived from the intercrossing of distinct individuals of the same 

 species. When distinct species are crossed the case is reversed, 

 for a plant's own pollen is almost always prepotent over foreign 

 pollen ; but to this subject we shall return in a future chapter. 



In the case of a large tree covered with innumerable flowers, it 

 may be objected that pollen could seldom be carried from tree to 

 tree, and at most only from flower to flower on the same tree ; 

 and flowers on the same tree can be considered as distinct indi- 

 viduals only in a limited sense. I 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 treos 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 



