The two volumes comprising the “Origin of Species” constitute, as the author said, one long argument.  It is, of course, impossible in the space at our command to recapitulate in detail even the leading facts and inferences which are brought forward to prove that species have been modified during a long course of descent.  We must confine ourselves to a succinct statement of the author’s general conclusions.  What he undertakes to prove is that the modification of species during a long course of descent has been effected chiefly through the natural selection of numerous successive slight favorable variations, aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner,—­that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us, in our ignorance, to arise spontaneously.  It should be observed that Darwin does not attribute the modification exclusively to natural selection.  What he asserts is:  “I am convinced that natural selection has been the main, but not the exclusive, means of modification.”  He submits that a false theory would hardly explain in so satisfactory a manner as does the theory of natural selection the several large classes of facts marshalled in the two volumes now under review.  If it be objected that this is an unsafe method of arguing, Darwin rejoins that it is a method usual in judging of the common events of life, and has often been used by the greatest natural philosophers.  The undulatory theory of light, for instance, has thus been arrived at; and the belief in the revolution of the earth on its own axis was, until lately, supported by scarcely any direct evidence.  It is no valid objection to the Darwinian theory of the origin of species that science as yet throws no light on the far higher problem of the essence or origin of life.  Neither has any one explained what is the essence of the attraction of gravity, though nobody now objects to following out the results consequent on this unknown element of attraction.

Why, it may be asked, did nearly all the most eminent naturalists and geologists until recently decline to believe in the mutability of species?  Darwin replies that the belief that species were immutable productions was almost unavoidable as long as the history of the world was thought to be of short duration.  Even now that we have acquired some idea of the lapse of time, men are too apt to assume without proof that the geological record is so perfect that it would have afforded plain evidence of the mutation of species if they had really undergone mutation.  The chief cause, however, of the once-prevalent unwillingness to admit that one species has given birth to other and distinct species is the fact that men are slow to admit great changes of which they do not see the steps.  The difficulty is the same which was experienced by many geologists when Lyell first insisted that long lines of inland cliffs had been formed and great valleys excavated, not by catastrophes, but by the slow-moving agencies which we see still at work.  The human mind cannot grasp the full meaning of the term of even a million years; cannot add up and perceive the full effects of many slight variations accumulated during an almost infinite number of generations.