Mr. Darwin, on the other hand, holds the orthodox view of the descent of all the individuals of a species not only from a local birthplace, but from a single ancestor or pair; and that each species has extended and established itself, through natural agencies, wherever it could; so that the actual geographical distribution of any species is by no means a primordial arrangement, but a natural result.  He goes farther, and this volume is a protracted argument intended to prove that the species we recognize have not been independently created, as such, but have descended, like varieties, from other species.  Varieties, on this view, are incipient or possible species:  species are varieties of a larger growth and a wider and earlier divergence from the parent stock; the difference is one of degree, not of kind.

The ordinary view—­rendering unto Caesar the things that are Caesar’s—­looks to natural agencies for the actual distribution and perpetuation of species, to a supernatural for their origin.

The theory of Agassiz regards the origin of species and their present general distribution over the world as equally primordial, equally supernatural; that of Darwin, as equally derivative, equally natural.

The theory of Agassiz, referring as it does the phenomena both of origin and distribution directly to the Divine will—­thus removing the latter with the former out of the domain of inductive science (in which efficient cause is not the first, but the last word)—­may be said to be theistic to excess.  The contrasted theory is not open to this objection.  Studying the facts and phenomena in reference to proximate causes, and endeavoring to trace back the series of cause and effect as far as possible, Darwin’s aim and processes are strictly scientific, and his endeavor, whether successful or futile, must be regarded as a legitimate attempt to extend the domain of natural or physical science.  For, though it well may be that “organic forms have no physical or secondary cause,” yet this can be proved only indirectly, by the failure of every attempt to refer the phenomena in question to causal laws.  But, however originated, and whatever be thought of Mr. Darwin’s arduous undertaking in this respect, it is certain that plants and animals are subject from their birth to physical influences, to which they have to accommodate themselves as they can.  How literally they are “born to trouble,” and how incessant and severe the struggle for life generally is, the present volume graphically describes.  Few will deny that such influences must have gravely affected the range and the association of individuals and species on the earth’s surface.  Mr. Darwin thinks that, acting upon an inherent predisposition to vary, they have sufficed even to modify the species themselves and produce the present diversity.  Mr. Agassiz believes that they have not even affected the geographical range and the actual association of species, still less their forms; but that every adaptation of species to climate, and of species to species, is as aboriginal, and therefore as inexplicable, as are the organic forms themselves.