1x THE DEBT OF SCIENCE TO DARWIN 467 
plants, hitherto inexplicable, are due to the necessity of keep- 
ing away “unbidden guests,” such as snails, slugs, ants, and 
many other kinds of animals, which would destroy the flowers 
or the pollen before the seeds were produced. When this 
simple principle is once grasped, it is seen that almost all the 
peculiarities in the form, size, and clothing of plants are to 
be thus explained, as the spines or hairs of the stem and 
branches, or the glutinous secretion which effectually pre- 
vents ants from ascending the stem, the drooping of. the 
flowers to keep out rain or to prevent certain insects from 
entering them, and a thousand other details which are de- 
scribed in Kerner’s most instructive volume. This branch of 
the inquiry was hardly touched upon by Darwin, but it is 
none the less a direct outcome of his method and his teaching. 
The Struggle for Existence 
But we must pass on from these seductive subjects to give 
some indication of the numerous branches of inquiry of which 
we have the results given us in the Origin of Species, but 
which have not yet been published in detail. The observa- 
tions and experiments on the relations of species in a state of 
nature, on checks to increase and on the struggle for existence, 
were probably as numerous and exhaustive as those on domes- 
ticated animals and plants. As examples of this we find 
indications of careful experiments on seedling plants and 
weeds, to determine what proportion of them were destroyed 
by enemies before they came to maturity ; while another set 
of observations determined the influence of the more robust 
in killing out the weaker plants with which they come into 
competition. This last fact, so simple in itself, yet so much 
overlooked, affords an explanation of many of the eccentrici- 
ties of plant distribution, cultivation, and naturalisation. 
Every one who has tried it knows the difficulty or impossi- 
bility of getting foreign plants, however hardy, to take care 
of themselves in a garden as in a state of nature. Wherever 
we go among the woods, mountains, and meadows of the 
temperate zone, we find a variety of charming flowers growing 
luxuriantly amid a dense vegetation of other plants, none of 
which seem to interfere with each other. By far the larger 
number of these plants will grow with equal luxuriance in 
