276 NATURAL HISTORY SOCIETY OF DUBLIN. 



says the specific distinctions are often founded on minute differences, 

 states that he never found any difficulty in identifying the forms he met 

 with with those of other countries by aid of dried specimens and figures, 

 and he enumerates a goodly catalogue. I myself have seen some exam- 

 ples from other parts of Europe. Nay, I may appeal to Dr. Wallich's 

 paper on Desmidiese, collected in Bengal,* 4 where he recognises, and is 

 able to name from their own special inherent intrinsic characters, se- 

 veral of the species belonging to Britain ; thus, not only from still more 

 widely remote localities, but under circumstances of climate greatly 

 varying from that in which the same species occur here. It is true 

 that, in regard to several of the forms which I should be disposed to 

 regard as abundantly distinct, Dr. Wallich would often combine several 

 of such into a single species, under a common specific designation; but 

 yet this does not militate against this part of my argument : for he 

 was still able to identify the forms by their intrinsic characters there 

 as here, although he holds a different view from that which I have 

 hitherto found myself compelled to adopt, as to the value of those 

 characters. 



Dr. "Wallich thinks, "that in these forms such differences as the 

 number of indentations, the acuteness or obtuseness of the teeth, the 

 number of spinous processes, and so forth, indicate mere accidental varia- 

 tions." But these very characters, thus succinctly recapitulated, accord- 

 ing to the degree and mode in which they are presented, are amongst 

 the most available holdpoints for the discrimination, not of species alone, 

 but also of genera. In what does a Micrasterias differ from a Euas- 

 trum, a Staurastrum from a Cosmarium, &c. &c, but in the mode 

 and way, the degree and extent, in which these characters, and charac- 

 ters such as these, are presented — not to speak of the various forms 

 within those genera which Dr. Wallich goes so far as to allow are 

 really good species. Dr. Wallich, for instance, calls such forms aa 

 Micrasterias rotata, and M. denticulata — Euastrum didelta, and JE. an- 

 satum — as in each case but varieties of a single species, &c. Why admit 

 certain denticulations, and incisions, and processes, and lobes in these 

 forms to be good specific marks, and then arbitrarily stop short, and disal- 

 low other characters of the same nature possessed by one of the disputed 

 forms, and not by the other, and which each refuses ^ to lend to the 

 other, and say they are of no value — although, so far as we know, the 

 species depending on them can be recognised wherever the two forms are 

 found in various countries of Europe, and in Bengal? 



Dr. Wallich believes that " such differences indicate mere acci- 

 dental varieties, handed down, no doubt, from parents to progeny in 

 the same locality, so long as physical conditions remain the same.'' If 

 certain external physical conditions be the cause of such minor indivi- 

 dual characters, and if dissimilar conditions will cause their oblitera- 

 tion or transference, how is it that, under all conditions in which 



Annals of Nat. Hist.," 3rd Ser., vol. v., pp. 184, 273. 



