Though I've put this tidbit up on the relevant page of the Victorian Web (recommended site) a long time ago, it cannot harm to relate it once more.
A careful verbal revision
From that republished version stems the famous quote:
"Those who cavalierly reject the Theory of Evolution as not being adequately supported by facts, seem to forget that their own theory is supported by no facts at all." (Spencer 1858, p. 389, capitals original)The original, however, read:
"Those who cavalierly reject the theory of Lamarck and his followers, as not adequately supported by facts, seem quite to forget that their own theory is supported by no facts at all."(go here and then click through to the facsimile of The Leader, no. 104, 20 March 1852, p. 280)The significance of the verbal revision
What's the significance of Spencer changing his wording at this peculiar time?* In the Preface (p. v) to his Essays, Spencer only said that all of the essays "have undergone careful verbal revision" and he signed that preface as follows: "St. John's Wood, December, 1857." That means, he changed his wording half a year before Wallace's letter arrived Down House, 18th June 1858, or before the Darwin & Wallace papers were jointly read before the Linnean Society, 1st July 1858 an.
Therefore these events cannot have had any causal role in Spencer's revision. It is more likely that Spencer tried to align his earlier essay, The Development Hypothesis (1852), with his own later essay, Progress: its Law and Cause (1857). In the latter essay Spencer tried to give a general and scientific definition of progress, that is, he wanted to be able to measure progress without recourse to human values.
"The current conception is a teleological one. The phenomena are contemplated solely as bearing on human happiness. Only those changes are held to constitute progress which directly or indirectly tend to heighten human happiness. And they are thought to constitute progress simply because they tend to heighten human happiness. But rightly to understand Progress, we must inquire what is the nature of these changes, considered apart from our interests." (p. 446 in: Spencer, H. 1857. "Progress: its Law and Cause. Westminster Review 67, 445-485, emphases original)In order to achieve this he started with organic progress as in the development of an organism and concluded:
"It is settled beyond dispute that organic progress consists in a change from the homogeneous to the heterogeneous. Now, we propose in the first place to show, that this law of organic progress is the law of all progress. Whether it be in the development of the Earth, in the development of Life upon its surface, in the development of Society, of Government, of Manufractures, of Commerce, of Language, Literature, Science, Art, this same evolution of the simple into the complex, through a process of continuous differentiation, holds throughout." (Spencer 1857, p. 446)Here, in a nutshell, Spencer has already written down the programme for his Synthetic Philosophy and linked his universal notion of evolution to a definition of progress he thought to be objective.
After so many examples illustrating processes that lead from homogeneous and undifferentiated states towards heterogeneous and differentiated states in organic, chemical, geologic, astronomic, economic, social, lingual and other systems, he arrives at a rather trivial explanation for this general trend:
"Every active force produces more than one change—every cause produces more than one effect." (Spencer 1857, p. 466, emphasis original)
"Starting with the ultimate fact that every cause produces more than one effect, we may readily see that throughout creation there must necessarily have gone on and must still go on, a never-ceasing transformation of the homogeneous into the heterogeneous." (Spencer 1857, p. 467)This law of progress would become Spencer's law of evolution stated in various places in his vast Synthetic Philosophy, for example in his First Principles (first published in 1864):
"Evolution is an integration of matter and concomitant dissipation of motion; during which the matter passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity." (Spencer 1900. First Principles. Appleton and Co., part 2, chap. 17, § 145, p. 367)Conclusion
We can therefore conclude that Spencer's careful verbal revision from "theory of Lamarck and his followers" to "Theory of Evolution" was not some premonition that Lamarckism would soon be replaced by Darwinism. Spencer simply worked on his project to generalise his own conceptions to arrive at his Synthetic Philosophy promoting a universal and progressive evolutionism.
* I've been prompted to answer that question by B. Ricardo Brown asking me in the first place