Having, towards the close of 1885, received an invitation from the Lowell Institute, Boston, U.S.A., to deliver a course of lectures in the autumn and winter of 1886, Wallace decided upon a series which would embody those theories of evolution with which he was most familiar, with a special one on “The Darwinian Theory” illustrated by a set of original diagrams on variation.  These lectures eventually became merged into the well-known book entitled “Darwinism.”

On the first delivery of his lecture on the “Darwinian Theory” at Boston it was no small pleasure to Wallace to find the audience both large and attentive.  One of the newspapers expressed the public appreciation in the following truly American fashion:  “The first Darwinian, Wallace, did not leave a leg for anti-Darwinism to stand on when he had got through his first Lowell Lecture last evening.  It was a masterpiece of condensed statement—­as clear and simple as compact—­a most beautiful specimen of scientific work.  Dr. Wallace, though not an orator, is likely to become a favourite as a lecturer, his manner is so genuinely modest and straightforward.”

Wherever he went during his tour of the States this lecture more than all others attracted and pleased his audiences.  Many who had the opportunity of conversing with him, and others by correspondence, confessed that they had not been able to understand the “Origin of Species” until they heard the facts explained in such a lucid manner by him.  It was this fact, therefore, which led him, on his return home in the autumn of 1887, to begin the preparation of the book ("Darwinism”) published in 1889.  The method he chose was that of following as closely as possible the lines of thought running through the “Origin of Species,” to which he added many new features, in addition to laying special emphasis on the parts which had been most generally misunderstood.  Indeed, so fairly and impartially did he set forth the general principles of the Darwinian theory that he was able to say:  “Some of my critics declare that I am more Darwinian than Darwin himself, and in this, I admit, they are not far wrong.”

His one object, as set out in the Preface, was to treat the problem of the origin of species from the standpoint reached after nearly thirty years of discussion, with an abundance of new facts and the advocacy of many new and old theories.  As it had frequently been considered a weakness on Darwin’s part that he based his evidence primarily on experiments with domesticated animals and cultivated plants, Wallace desired to secure a firm foundation for the theory in the variation of organisms in a state of nature.  It was in order to make these facts intelligible that he introduced a number of diagrams, just as Darwin was accustomed to appeal to the facts of variation among dogs and pigeons.

Another change which he considered important was that of taking the struggle for existence first, because this is the fundamental phenomenon on which Natural Selection depends.  This, too, had a further advantage in that, after discussing variations and the effects of artificial selection, it was possible at once to explain how Natural Selection acts.