Mr. Agassiz has “lost no opportunity of urging the idea that, while species have no material existence, they yet exist as categories of thought in the same way [and only in the same way] as genera, families, orders, classes,” etc.  He

“has taken the ground that all the natural divisions in the animal kingdom are primarily distinct, founded upon different categories of characters, and that all exist in the same way, that is, as categories of thought, embodied in individual living forms.  I have attempted to show that branches in the animal kingdom are founded upon different plans of structure, and for that very reason have embraced from the beginning representatives between which there could be no community of origin; that classes are founded upon different modes of execution of these plans, and therefore they also embrace representatives which could have no community of origin; that orders represent the different degrees of complication in the mode of execution of each class, and therefore embrace representatives which could not have a community of origin any more than the members of different classes or branches; that families are founded upon different patterns of form, and embrace, representatives equally independent in their origin; that genera are founded upon ultimate peculiarities of structure, embracing representatives which, from the very nature of their peculiarities, could have no community of origin; and that, finally, species are based upon relations—­and proportions that exclude, as much as all the preceding distinctions, the idea of a common descent.

“As the community of characters among the beings belonging to these different categories arises from the intellectual connection which shows them to be categories of thought, they cannot be the result of a gradual

material differentiation of the objects themselves.  The argument on which these views are founded may be summed up in the following few words:  Species, genera, families, etc., exist as thoughts, individuals as facts."[III-18]

An ingenious dilemma caps the argument: 

“It seems to me that there is much confusion of ideas in the general statement of the variability of species so often repeated lately.  If species do not exist at all, as the supporters of the transmutation theory maintain, how can they vary?  And if individuals alone exist, how can the differences which may be observed among them prove the variability of species?”

Now, we imagine that Mr. Darwin need not be dangerously gored by either horn of this curious dilemma.  Although we ourselves cherish old-fashioned prejudices in favor of the probable permanence, and therefore of a more stable objective ground of species, yet we agree—­and Mr. Darwin will agree fully with Mr. Agassiz—­that species, and he will add varieties, “exist as categories of thought,” that is, as cognizable distinctions—­which is all that we can make of the phrase here, whatever