After stating this more in detail than we have previously done, we shall be in a better position to judge if the process (which in the main we have no desire to deny or even to question) can dispense with guidance and the fixing of certain lines and limits within which, and of certain types towards which, the development proceeds. That is our point.
It is hardly necessary to illustrate the enormous destruction of life which goes on in the world. Even among the human race, the percentage of infants that die in the first months of their life is very large. But in the lower forms of life it is truly enormous. Only consider the myriads of insects that perish from hunger or accident, and from the preying of one species on another. If it were not so, the world would be overrun by plagues of mice, of birds, of insects of all kinds, and indeed by creatures of every grade. The term "struggle for existence" is, then, not an inapt one. All forms of living creatures have to contend with enemies which seek to prey upon or to destroy them, with the difficulty of obtaining food, and with what I may call the chances of nature—cold, storms, floods, disease, and so forth.
Now, it is obvious that if some creatures of a given kind possess some accidental peculiarity or modification in their formation which gives them (in one way or another) an advantage over their fellows, these improved specimens are likely to survive, and, surviving, to have offspring.
It is this perpetuation of advantageous changes, originally induced by the circumstances of environment, that is indicated by the term "natural selection." Nature chooses out the form best suited to the circumstances which surround it, and this form lives while the others die out. And this form goes on improving by slow successive changes, which make it more and more fit for the continually changing circumstances of its life.
Subordinate also to this natural selection is the principle that bright colour and other special qualities may be developed in the males of a race, because individuals with such advantages are more attractive, and therefore more easily find mates, than dull-coloured or otherwise less attractive individuals.
Of each of these principles I may give a simple example. Supposing a species of bird with a soft slender beak to be placed on an island, where the only food they could obtain was fruit enclosed in a hard or tough shell or covering. Supposing some birds accidentally possessed of a beak that was shorter and stouter than the others', these would be able to break open the shell and get at the fruit, while the others would starve. Some of the descendants of the birds with the stout beaks would inherit the same peculiarity, and in the course of several generations there would thus arise a species with short and strong, perhaps curved, beaks just fitted to live on fruits of the kind described. In a similar way the webbed feet of birds that swim were developed by their aquatic habits. And so with the long slender toes of the waders, which are so well fitted for walking over floating aquatic plants.
Of the other principle, sexual selection, a familiar example is the bright and showy colouring of the male birds of many species: the females of their species, as they need protection while helplessly sitting on their eggs, are dull-coloured like the bark of trees or the sand, among which their nests lie hid.
Some of the Himalayan pheasants exhibit this peculiarity to a marked degree. Originally, it is said, the male bird, which was more brightly coloured than the rest, got mated more easily by the preference shown to him for his bright colour.
The question is, can we suppose all this to go on, by self-caused laws and concurrence of circumstances, without a pre-existing design for the forms to reach or an external guidance in the processes?