It is a familiar experience to the student, not only of the Mollusca, but of every branch of animal or vegetable life, to come across examples which exhibit certain slight deviations from the type form as usually understood. These deviations may be more or less pronounced, but, as a rule, a series of forms can be discovered, gradually leading up to or down from the type. The definition of what constitutes a species,--and, still more, the rigid application of such definition--will always remain a difficult task, so long as the personal element persists in him who defines.[183] What seems to one authority ample ground for distinction of species, another may regard as of comparatively trivial importance. The practical outcome of these divergent views is sufficiently illustrated by the attitude of Mr. F. P. Marrat on the one hand, and of what may be called the modern French school of conchologists on the other. Mr. Marrat holds, or held, that the great genus _Nassa_, of which more than 150 species are generally recognised, is one shell (species) in an endless variety of forms. The modern French school go to the other extreme, and apparently proceed upon the view that almost any difference in form, however slight, is sufficient to constitute a separate species. It will be generally admitted, however, that some _structural difference_ in the organisation of the animal (as distinct from that of the shell alone) is necessary for the permanent constitution of specific rank.[184] What _amount_ of structural difference is required, what particular organ or organs must exhibit this difference, will depend largely upon the idiosyncrasy of the observer. But if this, or something like this definition of a species be accepted, it will follow that a so-called ‘variety’ will be a form which exhibits differences from the type which do not amount to permanent structural differences in the organisation of the animal. The final court of appeal as to what affords sufficient evidence for ‘permanent structural differences’ will have to be, as with Aristotle of old, the judgment of the educated man.