250 THE FOKMS OF CELLS [ch. 



In very many cases (of which Fig. 80 is an example), we have 

 an unduloid form exhibited, not by a surrounding pelhcle or shell, 

 but by the soft, protoplasmic body of a ciliated organism. In 

 such cases the form is mobile, and continually changes from one 

 to another unduloid contour, according to the movements of the 

 animal. We have here, apparently, to deal with an unstable 

 equilibrium, and also sometimes with the more comphcated 

 problem of " stream- lines," 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 treat- 

 ment 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 bio- 

 logical concept of organic species. In the intense classificatory 

 activity of the last hundred years, it has come about that every 

 form which is apparently characteristic, 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, — 

 for we need not concern ourselves with the occasional discussions, 

 or individual opinions, as to whether such and such a form deserve 

 "specific rank," or be "only a variety." And this secular labour 

 is pursued in direct obedience to the precept of the Systema 

 Naturae, — "ut sic in summa confusione reriim apparenti, siimmvs 

 conspiciatur Naturae ordo." In hke manner the physicist records, 

 and is entitled to record, his many hundred "species" of snow- 

 crystals*, or of crystals of calcium carbonate. But regarding 

 these latter species, the physicist makes no assumptions : he 

 records them simpliciter, as specific "forms"; 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 



* The case of the snow-crystals is a particularly interesting one; for their 

 "distribution" is in some ways analogous to what we find, for instance, among our 

 microscopic skeletons of Radiolarians. That is to say, we may one day meet 

 with myriads of some one particular form or species only, and another day with 

 myriads of another; while at another time and place we may find species inter- 

 mingled in inexhaustible variety. (Cf. e.g. J. Glaisher, III. London News, Feb. 17, 

 1855; Q.J. M.S. iii, pp. 179-185, 1855). 



