To us the present revival of the derivative hypothesis, in a more winning shape than it ever before had, was not unexpected.  We wonder that any thoughtful observer of the course of investigation and of speculation in science should not have foreseen it, and have learned at length to take its inevitable coming patiently; the more so as in Darwin’s treatise it comes in a purely scientific form, addressed only to scientific men.  The notoriety and wide popular perusal of this treatise appear to have astonished the author even more than the book itself has astonished the reading world.  Coming, as the new presentation does, from a naturalist of acknowledged character and ability, and marked by a conscientiousness and candor which have not always been reciprocated, we have thought it simply right to set forth the doctrine as fairly and as favorably as we could.  There are plenty to decry it, and the whole theory is widely exposed to attack.  For the arguments on the other side we may look to the numerous adverse publications which Darwin’s volume has already called out, and especially to those reviews which propose directly to refute it.  Taking various lines and reflecting very diverse modes of thought, these hostile critics may be expected to concentrate and enforce the principal objections which can be brought to bear against the derivative hypothesis in general, and Darwin’s new exposition of it in particular.

Upon the opposing side of the question we have read with attention, 1. an article in the “North American Review” for April last; 2. one in the “Christian Examiner,” Boston, for May; 3.  M. Pictet’s article in the “Bibliotheque Universelle,” which we have already made considerable use of, which seems throughout most able and correct, and which in tone and fairness is admirably in contrast with, 4. the article in the “Edinburgh Review” for May, attributed—­although against a large amount of internal presumptive evidence—­to the most distinguished British comparative anatomist; 5. an article in the “North British Review” for May; 6. finally, Professor Agassiz has afforded an early opportunity to peruse the criticisms he makes in the forthcoming third volume of his great work by a publication of them in advance in the “American Journal of Science” for July.

In our survey of the lively discussion which has been raised, it matters little how our own particular opinions may incline.  But we may confess to an impression, thus far, that the doctrine of the permanent and complete immutability of species has not been established, and may fairly be doubted.  We believe that species vary, and that “Natural Selection” works; but we suspect that its operation, like every analogous natural operation, may be limited by something else.  Just as every species by its natural rate of reproduction would soon fill any country it could live in, but does not, being checked by some other species or some other condition,—­so it may be surmised that