As a caution to the unwary, it should be remarked that the opposition supposed to exist between Mr. Darwin’s phrase ‘Natural Selection,’ and Mr. Herbert Spencer’s ‘Survival of the Fittest,’ is purely imaginary. The latter, no doubt, is the more philosophically accurate, the former is a convenient, popular, and telling metaphor. They both express the same conception of a large and wonderful group of facts. Perhaps it will be scarcely necessary to caution the unwary against taking for granted that ‘vestigiform structures are proofs of a typical formation;’ but if they are, they prove that in the typical formation of man a tail was included, which would be such a disgrace to the typical formation as would prevent all worthy and decorously-minded persons from believing in typical formations for a moment. DARWINISM, AND THE FIRST VERTEBRATE. Sir,--Your amiable and earnest correspondent does not seem to understand that men like Darwin and Wallace, who have spent years of patient labour and thought in amassing observations of nature, and grouping together the facts out of which their theories have been formed, have a right to ‘an air of philosophical superiority,’ if they choose to display it, when questions are asked or arguments put forward which imply ignorance or misconception of all they have been doing in the interests of science. When Mr. Wallace ‘very coolly’ asserts that he sees no force in an argument, it will as a rule be advisable for the argument either to withdraw itself from the public gaze, or get itself stated a little more lucidly. Again, when Mr. Darwin ‘rashly affirms that he has distinct evidence’ of a thing, it would perhaps be a good plan to get the Commissioners in Lunacy to examine the astonishing number of hard-headed men whom he induces to believe his unfounded assertion.