ix 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 



