OF INDIVIDUALS. 87 



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 

 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 individuals 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 tend- 

 ency 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 re- 

 marks 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 indispensable; for owing to the nature of the 

 fertilizing element there are no means, analogous to th9 



