NATURAL SELECTION. I43 
STRUGGLE FOR EXISTENCE. 
Every one knows that the life of an individual plant or 
animal depends on a proper supply of food, is affected by 
changes of climate, and is constantly endangered by dis- 
ease and enemies ; few are, however, aware of the extent 
to which individual life is dependent on the existence of 
some other kind of life, and of the extremely complex 
nature of the struggle for existence. Thus, according to 
Prof Haeckel, “There are small oceanic islands whose 
inhabitants live essentially on a species of Palm. The 
fructification of these Palms is effected principally through 
Insects, who carry the pollen from the male to the female 
Palm. The existence of these useful Insects is endangered 
through Insect-feeding birds, who in turn are pursued by 
Rapacious birds. But the Rapacious birds often succumb 
under the attacks of a small parasitic Mite, which develops 
by millions in their feathery coats. These small, dangerous 
Parasites can be killed through parasitic Fungi. Fungi, 
Rapacious birds, and Insects in this case would favor, Bird- 
mites and Insect-feeding birds, on the contrary, would 
endanger, the growth of the Palms, and consequently of 
men.” Thus the existence of entire populations may be 
indirectly dependent on the presence of a highly insignifi- 
cant plant or animal form. If one considers the millions 
of eggs laid by fishes, and that a pair of elephants, the 
slowest of breeders, would reproduce in five hundred years 
fifteen millions, the importance of the struggle for existence 
in checking over-population will be appreciated. Accord- 
ing to Mr. Darwin, the red clover never produces seed if 
the humble-bees be prevented from visiting it. For the 
bee, in sucking the honey out of the flower, brings the 
pollen in contact with the pistil, and by this means the 
clover is fertilized. Now, it is well known that the bees 
are destroyed by the field-mice, and that the number of 
