82 THE VOYAGE OF H.M.S. CHALLENGER, 



nor those of his opponents can be adopted without amendments. As to the suggestion 

 that generic distinctions must be of an absolute character, of course the acquiescence in 

 this demand would place the creation of genera beyond the discretion of classifiers, and 

 thus expel for ever from Science the disputes as to whether this group is to be regarded 

 as a species or as a genus ; it is however evident that, strictly and exactly prosecuted, 

 this demand would lead to the most strange and unnatural systematic arrangements. 

 Following it we should be obliged to unite all the Keratosa, with the exception of the genera 

 lanthella, Darwinella, and perhaps Psammopemma with all Chalinidse, Renierid^, perhaps 

 all Monactinellida into a single genus systematically equivalent to that of Danoiniella or 

 lanthella. For there are no absolute distinctions between Chalina or Reniera and 

 Spongelia, and there are no absolute distinctions between Spongelia and Etispongia and 

 Aplysina, and again there exist such distinctions between most of the Keratosa and ]\Ionac- 

 tinellida on one hand, and lanthella, Darwinella, and Psammopemma on the other. On the 

 whole, what Prof. Nageli recommends is applicable only to the palgeontologically old groups 

 of plants and animals. But it is not less evident that an unlimited discretion as to the 

 creation of genera would render any systematic progress impossible; to those who feel 

 disinclined to agree with me, I can but recommend an attentive perusal of spongiological 

 systematic literature. The using of a varietal character, as of generic value, condi- 

 tions the establishment of new apparently highly interesting and deviating species 

 out of forms representing nothing more than by no means instructive varieties, 

 if not individuals of very common species belonging to another genus. It is quite 

 possible that I have myself committed the same mistake, having adopted F, E. Schulze's 

 genus Hippospongia, and created a new species H.ipp)ospongia Tnauritiana, while this 

 conjectural species is very possibly nothing but a variety or subspecies of Spongia 

 (Euspongia) lapideseens. Both these opinions cannot thus in their entire extension be 

 adopted. In my paper on the Challenger Calcarea an attempt to reconcile them has 

 been made. Whilst subdividing this group into genera I called attention to my 

 intention to execute this task according to the whole of their organisation, by taking 

 into consideration aU their organs in their mutual correlation ; and with respect to 

 the group just mentioned the carrying out of this principle met no difficulties, 

 and I hope that the genera I have there established are really natural and adoptable. 

 But there are very often cases when such a proceeding is impossilile, when large groups 

 of forms differ from one another only in a single character, the conjectural specific 

 distinctions being in one group complete analogues to those in the other. To similar 

 instances the principle I have followed in my above named paper is not applicable, and for 

 my own part I see no other issue but to recommend for such cases the adoption of the scheme 

 of Nageli, and this in order to protect the establishment of genera so far as possible from 

 the vagaries of classifiers, so that generic unity might serve as a firm basis, which has 

 been wanting in descriptive zoology since the mutability of species was actually proved. 



