changes from one to another unduloid contour, according to the movements of the animal. We have here, apparently, to deal with an unstable equiôÙlibôÙrium, and also sometimes with the more complicated problem of ãstream-lines,ã as in the difficult problems suggested by the form of a fish. But this whole class of cases, and of problems, we can merely take note of in passing, for their treatment is too hard for us.
In considering such series of forms as the various unduloids which we have just been regarding, we are brought sharply up (as in the case of our Bacteria or Micrococci) against the biological concept of organic species. In the intense clasôÙsiôÙfiôÙcaôÙtory activity of the last hundred years, it has come about that every form which is apparently charôÙacôÙterôÙisôÙtic, that is to say which is capable of being described or portrayed, and capable of being recognised when met with again, has been recorded as a species,ãfor we need not concern ourselves with the occasional discussions, or individual opinions, as to whether such and such a form deserve ãspecific rank,ã or be ãonly a variety.ã And this secular labour is pursued in direct obedience to the precept of the Systema Naturae,ããut sic in summa confusione rerum apparenti, summus conspiciatur Naturae ordo.ã In like manner the physicist records, and is entitled to record, his many hundred ãspeciesã of snow-crystals36, or of crystals of calcium carbonate. But regarding these latter species, the physicist makes no assumptions: he records them simpliciter, as specific ãformsã; he notes, as best he can, the circumstances (such as temperature or humidity) under which they occur, in the hope of elucidating the conditions determining their formation; but above all, he does not introduce {251} the element of time, and of succession, or discuss their origin and affiliation as an historical sequence of events. But in biology, the term species carries with it many large, though often vague assumptions. Though the doctrine or concept of the ãpermanence of speciesã is dead and gone, yet a certain definite value, or sort of quasi-permanency, is still connoted by the term. Thus if a tiny foraminiferal shell, a Lagena for instance, be found living to-day, and a shell inôÙdisôÙtinôÙguishôÙable from it to the eye be found fossil in the Chalk or some other remote geological formation, the assumption is deemed legitimate that that species has ãsurvived,ã and has handed down its minute specific character or characters, from generation to generation, unchanged for untold myriads of years37. Or if the ancient forms be like to, rather than identical with the recent, we still assume an unbroken descent, accompanied by the hereditary transmission of common characters and progressive variations. And if two identical forms be discovered at the ends of the earth, still (with occasional slight reservations on the score of possible ãhomoplasyã), we build hypotheses on this fact of identity, taking it for granted that the two appertain to a common stock, whose dispersal in space must somehow be accounted for, its route traced, its epoch determined, and its causes discussed or discovered. In short, the naturalist