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 it may mean in the Aristotelian metaphysics.  Admitting that species are only categories of thought, and not facts or things, how does this prevent the individuals, which are material things, from having varied in the course of time, so as to exemplify the present almost innumerable categories of thought, or embodiments of Divine thoughts in material forms, or—­viewed on the human side—­in forms marked with such orderly and graduated resemblances and differences as to suggest to our minds the idea of species, genera, orders, etc., and to our reason the inference of a Divine original?  We have no clear idea how Mr. Agassiz intends to answer this question, in saying that branches are founded upon different plans of structure, classes upon different modes of execution of these plans, orders on different degrees of complication in the mode of execution, families upon different patterns of form, genera upon ultimate peculiarities of structure, and species upon relations and proportions.  That is, we do not perceive how these several “categories of thought” exclude the possibility or the probability that the individuals which manifest or suggest the thoughts had an ultimate community of origin.  Moreover, Mr. Darwin would insinuate that the particular philosophy of classification upon which this whole argument reposes is as purely hypothetical and as little accepted as his own doctrine.  If both are pure hypotheses, it is hardly fair or satisfactory to extinguish the one by the other.  If there is no real contradiction between them, there is no use in making the attempt.

As to the dilemma propounded, suppose we try it upon that category of thought which we call chair.  This is a genus, comprising the common chair, (Sella vulgaris,) the arm or easy chair, (S. cathedra,) the rocking chair, (S. oscillans,) widely distributed in the United States, and some others,—­each of which has sported, as the gardeners say, into many varieties.  But now, as the genus and the species have no material existence, how