The common view of species is, that, although they are generalizations, yet they have a direct objective ground in Nature, which genera, orders, etc., have not.  According to the succinct definition of Jussieu,—­and that of Linnaeus is identical in meaning,—­a species is the perennial succession of similar individuals in continued generations.  The species is the chain of which the individuals are the links.  The sum of the genealogically connected similar individuals constitutes the species, which thus has an actuality and ground of distinction not shared by genera and other groups which were not supposed to be genealogically connected.  How a derivative hypothesis would modify this view, in assigning to species only a temporary fixity, is obvious.  Yet, if naturalists adopt this hypothesis, they will still retain Jussieu’s definition, which leaves untouched the question as to how and when the “perennial successions” were established.  The practical question will only be, How much difference between two sets of individuals entitles them to rank under distinct species; and that is the practical question now, on whatever theory.  The theoretical question is—­as stated at the beginning of this long article—­whether these specific lines were always as distinct as now.

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."[11]