392 Mr. A, H. Cooke on the 



cularly the former, as including genera which, as genera go, are 

 fairly constant in form. And further, if the reason Issel gives 

 for distinguishing ''equivalent varieties" from "equivalent 

 species " be a valid one, it ought to be true for the one set 

 of genera but inoperative with regard to the other; in other 

 words, his " equivalent varieties " ought to belong to genera 

 which are markedly variable, while his " equivalent species " 

 ought to belong to genera which are fairly constant. So far 

 is this from being the case, that half of the genera which 

 supply him with " equivalent varieties ^' supply '' equivalent 

 species" as well. Why is Area auri'culaia the ''equivalent 

 species " of Area dihivn, but Aoxa erythrcea only the " equi- 

 valent variety " of Ai-ca lactea ? Why, again, is Nassa 

 circttmcineta the '' equivalent species " of N. gihbosxda^ while 

 N. costulata (Ren., = variabiNs, Forb.) of the Mediterranean 

 has only an " equivalent variety " in the Red Sea? Again, 

 what genera are more notoriously liable to variation than 

 Tojyes, Modiola, and Purpura ? Yet all the Red-Sea " equi- 

 valents " of Mediterranean species of these genera are classed 

 by Issel as species, not as varieties. The reply will be, that 

 it is a question of greater or less modification of form. But 

 who is to measure " greater or less " ? And what has become 

 of the " groups of genera which are subject to considerable 

 variation of form " ? 



These terms, " a variable genus," a genus " liable to 

 variation," seem to me misleading, because they attempt to 

 cover too much ground, but at the same time do not grasp 

 all the facts as they present themselves to us. What one 

 notices is, that certain genera, from their mode of life and 

 habitat, are, so to speak, compelled to vary. Such genera, 

 including Ostrea, Vulsella, Chama, Avi'eula, Littorina, Ver- 

 metuSy Crepidula, Patella, &c., I should term genera neces- 

 sarily variable. Again, one notices that certain other genera, 

 while not necessarily variable, possess individual species 

 which exhibit capacities for variation, while the bulk of the 

 species remain fairly constant to the type. Such genera are : — 

 Conus, fairly constant as a whole, but exhibiting such ex- 

 tremely variable species as textile and Jiycena ; Natica, with 

 the variable maroeeana and mamilla ; Nassa, with the 

 variable gaudiosa and variabilis ; Cardium, with the variable 

 edule. Thus, while a variable genus implies variation in the 

 subordinate species, a variable species by no means implies a 

 variable genus. W^hy some species should vary while others 

 remain constant to the type is as yet unknown, but the fact is 

 unquestionable. 



It appears to me, then, that Issel's refusal of the terra 



