384 REV. GEORGE HENSLOW ON THE 
the general rule appears to be that as long as the pairs of seedlings planted in the same 
pot in his experiments were free from competition there was not much difference in their 
growth. As soon, however, as they began to compete, the crossed plants showed their 
superior vigour, and so checked the growth of their self-fertilized competitors. They 
would inevitably continue to do so by abstracting the nourishment which the others 
would otherwise have secured, and so would ultimately, as was so generally the case, beat 
the self-fertilized plants (7. c.[p. 285). Secondly, in some of Mr. Darwin’s experiments he 
grew the plants much more crowded in some pots than in others. In these latter the 
ratio of the heights of the intercrossed to the self-fertilized were almost invariably 
nearer unity than that deduced from the crowded plants. Lastly, when the plants 
were grown in open ground, and entirely free from competition, there was often, as 
Mr. Darwin states, but little difference in their growth *. 
Mr. Darwin planted pairs in the same pot in order to imitate nature ; but the struggling 
between individuals of the same species is not generally quite so common as that between. 
different genera; and though his experiments bring out well enough the superiority of 
the crossed to the self-fertilized under competition, they fail to show what would be 
their relative values when competing with other genera of other orders. It is a well- 
known fact that plants of different orders, and presumably of different constitutions, not 
requiring identically the same food, can live and flourish together or in succession when 
plants of the same species would fail to do so. This is seen in the flora of oceanic 
islands, in which the number of orders is large in comparison with the number of genera, 
and the latter large as compared with the number of species. It is also the basis of the 
principle. upon which the rotation of crops is founded; but for Mr. Darwin’s object 
perhaps the method adopted was the best. 
The question, however, arises, Why is it, if the intercrossed are so superior in vigour 
to the self-fertilized, that such a difference is noć so great when they are free from com- 
petition? If there were any evil or injurious effects, as Mr. Darwin supposes, to follow 
from self-fertilization, it is difficult to understand why the ratio does not keep constant 
under all circumstances; for an ample space and a good soil being common to both, 
one would, à priori, infer that the intercrossed would be benefited thereby just as much 
as, if not even in a higher degree than, the self-fertilized. This, however, does not 
appear to be the case. A few examples from Mr. Darwin’s experiments will illus- 
trate this. T 
. In several of his tables he has written ** plants crowded” to certain pots; and I find 
by calculating the ratios of these, as well as the ratios of all the rest not so crowded, 
that they are, with a solitary exception, nearer unity in the less crowded. Thus, omittin| j 
decimals :— ey 
Ipomea purpurea, Table-X., not crowded. sT sar sor «..s + 4.100: 90. 
b h crowded < recta i fap oc cx MN A EM 
2 purpurea crossed on same plant, Table XII., not crowded. 100 : 111 
» e e crowded. . 100: 97 
Ditto crossed NEC Colchester enge? Table XIL, not crowded . 100: 83 
5. ean. »  . erowd 
s d ; “+ Animals and Plants undor Domest p. 128 
