Lamarck’s hypothesis of change produced by the attempts of animals to increase the development of their own organs has been often refuted, but the view here proposed depends upon the action of principles constantly working in nature. Retractile talons of falcons and cats have not been developed by volition, but by _the survival of those which had the greatest facilities for seizing prey_. The long neck of the giraffe was not produced by constant stretching, but by the success which any increase in the length of neck ensured to its possessors. Even colours, especially of insects, are explained in the same way, for among the varieties of many tints, those “having colours best adapted to concealment ... would inevitably survive the longest.” We can similarly explain deficiency of some organs with compensating development of others, “great velocity making up for the absence of defensive weapons,” etc. Varieties with an unbalanced deficiency could not long survive. The action of the principle is like the governor of a steam-engine, checking irregularities almost before they become evident. Such a view accords well with “the many lines of divergence from a central type”; the increasing efficiency of a particular organ in a series of allied species; the persistence of unimportant parts when important ones have changed; the “more specialised structure,” said by Owen to be characteristic of recent as compared with extinct forms. Hence there is a tendency of certain classes of _varieties_ to progress further and further from the original type, and there is no reason for assigning any limit to this progression. Such gradual changes “may, it is believed, be followed out so as to agree with all the phenomena presented by organised beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.” Wallace’s Essay has been reprinted without any alteration in his “Essays on Natural Selection,” recently re-issued combined with “Tropical Nature.” CHAPTER XI. COMPARISON OF DARWIN’S AND WALLACE’S SECTIONS OF THE JOINT MEMOIR--RECEPTION OF THEIR VIEWS--THEIR FRIENDSHIP. [Sidenote: WALLACE AND DARWIN.] Comparing the essays of these two naturalists, we observe that Darwin here first makes public the phrase “natural selection,” Wallace the “struggle for existence”; although so closely do their lines of thought converge that Darwin, using practically the same words, speaks of the “struggle for life.” Both show, by examples, the tendency of all animals to multiply at an enormous rate, and both show that their tolerably constant numbers are due to the constant supply of food.