to them in that well-known and powerful essay on ’The Development Hypothesis.’ ...  In the ‘Principles of Psychology,’ the first edition of which was published in 1855, the evolutionary principle was dominant.  By 1858—­the year of the announcement of Natural Selection by Darwin and Wallace—­he had conceived the great general scheme and had sketched out the first draft of the prospectus of the Synthetic Philosophy, the final and amended syllabus [being] issued in 1860.  The work of Darwin and Spencer from that period, although moving along independent lines, was directed towards the same end, notwithstanding the diversity of materials which they made use of and the differences in their methods of attack; that end was the establishment of Evolution as a great natural principle or law."[26]

In this connection it is especially interesting to note how near Spencer had come to the conception of Natural Selection without grasping its full significance.  In an article on a “Theory of Population” (published in the Westminster Review for April, 1852) he wrote:  “And here, indeed, without further illustration, it will be seen that premature death, under all its forms and from all its causes, cannot fail to work in the same direction.  For as those prematurely carried off must, in the average of cases, be those in whom the power of self-preservation is the least, it unavoidably follows that those left behind to continue the race must be those in whom the power of self-preservation is the greatest—­must be the select of their generation.  So that whether the dangers of existence be of the kind produced by excess of fertility, or of any other kind, it is clear that by the ceaseless exercise of the faculties needed to contend with them, and by the death of all men who fail to contend with them successfully, there is ensured a constant progress towards a higher degree of skill, intelligence, self-regulation—­a better co-ordinance of actions—­a more complete life.”

Up to the period of the publication of the “Origin of Species” and the first conception of the scheme of the Synthetic Philosophy there had been no communication between Darwin and Spencer beyond the presentation by Spencer of a copy of his Essays to Darwin in 1858, which was duly acknowledged.  But by the time the “Origin of Species” had been before the public for eight years, the Darwinian principle of selection had become an integral part of the Spencerian mechanism of organic evolution.  Indeed the term “survival of the fittest,” approved by both Darwin and Wallace as an alternative for “natural selection,” was, as is well known, introduced by Spencer.