162 BOTANY 
existing to prevent self and ensure cross-pollination, 
we come to the conclusion that the latter has some 
special advantage over the former. There is always 
a possible advantage in having two parents instead of 
only one, as would be the ease in self-pollination. The 
offspring in the latter case would reproduce very 
closely the constitution and peculiarities of the parent 
plant, and variations would be slight and infrequent. 
Where, however, the pollen is brought from the 
anthers of one flower to the stigma of another the 
offspring, having two parents, may show considerable 
variation from both. Its characteristics will be a 
mixture of those of its ancestors. In some instances, 
the new plant will produce the weak points of both 
parents, in which case it will be eliminated in the 
struggle for existence, in others again it will combine 
some of the strong and some of the weak points of 
both; and here, too, it will, though after a longer time, 
also be eliminated: finally there will now and then 
arise individuals which combine most of the strong 
points of one parent with the strong points of the 
other, and these are the plants that will eventually 
oust all competitors and obtain possession of the earth. 
It must be remembered that when we speak of strong 
points we mean those features that specially adapt the 
plant to its own particular environment. 
Darwin made a number of experiments to deter- 
mine the advantage of cross-pollination. He found 
that in certain plants growing close together cross- 
pollination secured in the offspring an advantage of 30. 
per cent. in vigour and productiveness over those 
resulting from self-pollination, and that, when this was 
carried on to the tenth generation, the advantage was 
increased to 50 per cent. By bringing pollen from a 
distant garden, even higher percentages were obtained. 
It 1s alleged that there are some flowers that are not 
fertile to their own pollen, but recent investigations 
