Dr. G. C. Wallich on the Diafomacea. 11 



accommodate themselves to the varying, but mighty, agencies of 

 the ocean. 



The tendency of the Diatom to approach the source of light 

 and heat is fully understood. It shares that tendency with all 

 other vegetable sti-uctures, although, from the fixed nature of 

 the higher orders, the effects are perhaps not so palpably recog- 

 nized. But there exists, it appears to me, quite sufficient evi- 

 dence to prove that this quality is entirely distinct from, and 

 independent of, the peculiar motile power upon which its animal 

 character was so long and so erroneously maintained. The one 

 phffinomenou is simply the result of those physical conditions 

 by which the growth and increase of the organism are determined ; 

 the other depends upon peculiarities of structure or function 

 which, although imperfectly understood, must nevertheless be 

 considered as imparted to it for the purpose of bringing it into 

 contact with fresh portions of the medium it inhabits, or of 

 enabling it to accommodate itself to the requirements of the 

 conjugativc process. 



Last year, among the Channel Islands, it was my good for- 

 tune to meet with a repetition of the phsenomenon witnessed by 

 me in the Indian seas. As in the former instance, my attention 

 was first attracted by the occm'rence, dui'ing calm spring weather, 

 of large frustules of a Coscinodiscus, \\i. C. concinnus. The sur- 

 face and depths, as far as the eye could pierce from the gunwale 

 of a boat, were thronged with the brilliant, glistening cylinders 

 of this Diatom, intermixed with filaments of Biddulphia Baileyii, 

 and, more sparingly, with the long acicular threads of lihizo- 

 selenia and Chatoceros. The self-buoyant property of these 

 coast-frequenting forms, although of a temporary character 

 only, is nevertheless evident, and indicates that they hold a po- 

 sition intermediate between the perpetually free-floating species 

 of the open sea and the subparasitic species of the fresh water, 

 which, although capable of self-support for brief periods, are 

 generally to be found in the neighbourhood of aquatic plants 

 or other objects. 



As it happens, Coscinodiscus concinnus, Biddulphia Baileyii, 

 and the several species of Rhizoselenia are amongst those forms 

 sometimes designated as " imperfectly silicious.^^ Their distinct- 

 ness as species can, however, in nowise be influenced by this 

 character. The quality of the silicious framework is, after all, 

 resolvable into a question, not of imperfect (that is, impure) 

 silicious secretion, but of the relative thickness and solidity of 

 that secretion in these as compared with other forms. If it 

 be for a moment contended that the silicious structure is per- 

 meated by any structure of vegetable origin, whether proto- 

 plasm or endochrome,' or cellulose, the only consistent view of 



