RANGE OF VARIATION. DIATOMACEvE. 273 



biradiatum, whose frond is normally composed of sixteen cells ; 

 whilst at a is figured a form which is designated as P. tetras, but 

 which may be strongly suspected to be merely a 4-celled variety of 

 B and c. Many similar cases might be cited ; and the Author 

 would strongly urge those Microscopists who have the requisite 

 time and opportunities, to apply themselves to the determination 

 of the real species of these groups, by studying the entire life 

 history of whatever forms may happen to lie within their reach, 

 and noting all the varieties which present themselves among the 

 offsets from any one stock. It must not be forgotten that this 

 process' of multiplication is analogous to the propagation of the 

 higher Plants by Gemmation or ' budding, 1 and to the subsequent 

 separation of the buds, either spontaneously, or by the artificial 

 operations of grafting, layering, &c. ; and just as in all these cases 

 the particular variety is propagated, whilst only the characters of 

 the species are transmitted by the true Generative operation to the 

 descendants raised from Seed, so does it come to pass that the cha- 

 racters of any particular variety which may arise among these 

 Unicellular Plants, are diffused by the process of binary subdivi- 

 sion amongst vast multitudes of so-called individuals. Thus it 

 happens that, as Mr. Ralfs has remarked, "one pool may abound 

 with individuals of Staurastrum dejectum or Arthrodesmus incus, 

 having the mucro curved outwards ; in a neighbouring pool, every 

 specimen may have it curved inwards ; and in another it may be 

 straight. The cause of the similarity in each pool no doubt is, 

 that all its plants are offsets from a few primary fronds." Hence 

 the universality of any particular character, in all the specimens 

 of one gathering, is by no means sufficient to entitle these to take 

 rank as a distinct species ; since they are, properly speaking, but 

 repetitions of the same form by a process of simple multiplication, 

 really representing in their entire aggregate the one Plant or Tree 

 that grows from a single seed. 



211. Diatomacece. — Notwithstanding the very close affinity 

 which, as will be presently shown, exists between this group and 

 the Desmidiacece, some Naturalists who do not hesitate in regard- 

 ing the members of the last-named family as Plants, persist in 

 referring the Diatomacece to the Animal kingdom. For this sepa- 

 ration, however, no adequate reason can be assigned ; the curious 

 movements which the Diatomacece exhibit being certainly not of 

 a nature to indicate the possession of any truly Animal endowment, 

 and all their other characters being unmistakably Vegetable. 

 Like the Desinidiaceae they are simple Cells, having a firm external 

 coating, within which is included a mass of endochrome whose 

 superficial layer seems to be consolidated into a sort of primordial 

 utricle. The external coat is consolidated by silex, the presence of 

 which in this situation is one of the most distinctive characters of 

 the group ; but it is a mistake to suppose that the casing is com- 

 posed of silex alone. For a Membrane bearing all the markings 



T 



