THE DISPERSAL 01' SEEDS. 
107 
Of the sexual processes, on the other hand, the essence is that the 
odspring owes its being to ^zro individual organisms, being developed from 
a cell of the one under the stimulus of fusion with the active principle of a 
cell of another, so that it may naturally be expected to partake — at least 
to some extent — of the characteristics of both its parents. Now, it may 
well be, as suggested by my friend Mr. Thomas Meehan, of Philadelphia, 
that, while all merely re]:)roductive functions could be quite adequately 
performed by vegetative processes, the great object of sex may be the 
introduction, by the competing influences of the two parents, of variation, 
i.e. the predominance of characters derived now from one, now from the 
other, and now from both parents. 
Now, the severest competition for the necessaries of life — food, air, 
and light — will be between organisms which, being nearly identical in 
structure and physiology, are also nearly identical in their require- 
ments. In other words, the struggle for existence will, among plants, be 
particularly keen among what we term " social " plants and where vege- 
tation is dense, and this competition will present the most favourable 
opportunity for the self-assertion of any variation. At the same time an 
organism may obviate much of the danger of home competition if it can 
secure removal to some other area — " to fresh woods and pastures new." 
For this purpose a short journey may suffice. A few hundred yards or 
less may bring fruit or seed to the newly-upturned soil of a railway 
bank, a ploughed field, or a garden border ; and, though we do find 
structures which may suffice, under exceptional circumstances, to carry 
them further afield, the direct primary adaptations for dispersal seem to 
be all mainly calculated for small distances. The well-filled fruitlet of 
the Thistle, for example, w^hen the fruit-head has been pulled to pieces 
by the goldfinch or other small bird, will not have floated far before the 
heavy ovary and the seed it contains will have detached itself from the 
ring which bears the parachute of down, and will have fallen to the earth, 
whilst the specimens of Thistle-down that come floating down from the 
sky in the heart of London will be found to be light, abortive fruits. The 
variously-shaped and twisted double fans of the Maples, which exhibit 
all the varied features of the screw-propellers of our ocean steamers, may 
be torn, wdien just ripe, from the tree by high wind and carried a con- 
siderable distance ; but if they merely fall to the ground, when ripe, in a 
dead calm, their structure is such as to produce a rapid gyration which 
will at least carry them beyond the shadow of the parent tree. While 
some of these, and similar, samaroid fruits and winged seeds have a spiral 
twist, in almost every case the seed is eccentric, which serves to give the 
initial twirl in the fall. If, again, an animal in passing brush against 
the ripe fruit of the Squirting Cucumber {Ecballium Elateriuin), one 
layer in the wall of which is in a state of tension from turgescence, the 
fruit will detach itself from its stalk and eject its contents violently from 
the opening thus made. The pulp-covered seeds may then adhere to the 
coat of the animal, and be so carried to a distance ; but the main purpose 
of the adaptation would seem to be accomplished by their being thrown 
to the distance of a few feet. 
Before saying more as to special adaptations to dispersal, it may bo 
well to point out the numerous cases in which seeds may be moat 
D 
