For obvious reasons such doctrines are likely to find more favor with botanists than with zoologists.  But with both the advance in this direction is seen to have been rapid and great; yet to us not unexpected.  We note, also, an evident disposition, notwithstanding some endeavors to the contrary, to allow derivative hypotheses to stand or fall upon their own merits—­to have indeed upon philosophical grounds certain presumptions in their favor—­and to be, perhaps, quite as capable of being turned to good account as to bad account in natural theology.[IV-3]

Among the leading naturalists, indeed, such views—­taken in the widest sense—­have one and, so far as we are now aware, only one thoroughgoing and thoroughly consistent opponent, viz., Mr. Agassiz.

Most naturalists take into their very conception of a species, explicitly or by implication, the notion of a material connection resulting from the descent of the individuals composing it from a common stock, of local origin.  Agassiz wholly eliminates community of descent from his idea of species, and even conceives a species to have been as numerous in individuals and as wide-spread over space, or as segregated in discontinuous spaces, from the first as at the later period.

The station which it inhabits, therefore, is with other naturalists in no wise essential to the species, and may not have been the region of its origin.  In Agassiz’s view the habitat is supposed to mark the origin, and to be a part of the character of the species.  The habitat is not merely the place where it is, but a part of what it is.

Most naturalists recognize varieties of species; and many, like De Candolle, have come to conclude that varieties of the highest grade, or races, so far partake of the characteristics of species, and are so far governed by the same laws, that it is often very difficult to draw a clear and certain distinction between the two.  Agassiz will not allow that varieties or races exist in Nature, apart from man’s agency.

Most naturalists believe that the origin of species is supernatural, their dispersion or particular geographical area, natural, and their extinction, when they disappear, also the result of physical causes.  In the view of Agassiz, if rightly understood, all three are equally independent of physical cause and effect, are equally supernatural.

In comparing preceding periods with the present and with each other, most naturalists and palaeontologists now appear to recognize a certain number of species as having survived from one epoch to the next, or even through more than one formation, especially from the Tertiary into the post-Tertiary period, and from that to the present age.  Agassiz is understood to believe in total extinctions and total new creations at each successive epoch, and even to recognize no existing species as ever contemporary with extinct ones, except in the case of recent exterminations.