W. D. Lang — On Stomatopora antiqua. 



2 Gr- 



and the Pliensbachian mutation are repi'esented by Figs. 1 and 2 in 

 Diagram I, on this page. 



Diagram I. — To show the changes in the method of branching and in the shape of 

 the zocecium in Stomatopora antiqua, Haime, and its related forms. 

 Fig. 1. — Stomatopora Gregoryi, sp. nov. 

 ,, 2. — Stomatopora Gregoryi, mut. capricornensis. 

 ,, 3. — Stomatopora antiqua, HBhae. 

 , , 4. — Sto^natopora antiqua, Haime, mut. capricornensis. 

 Figs. 5 and 6. — Stomatopora Richardsoni, sp. nov. 



Note. — A difficulty arises in the system of nomenclature advocated 

 above, wherein the mutation of a species is named as such as if it 

 were a variety; but with this difference, that, while a variety is 

 named arbitrarily, just as a species, the mutation is named according, 

 to the horizon at which it occurs. The difficulty is met when the 

 mutations of a given species pass upwards into a form already 

 named. For suppose in the case above it could be demonstrated 

 that S. Gregoryi passed upwards through its mutation capricornensis 

 and other possible mutations into the Aalenian species S. dichotomoides 

 (d'Orbigny), it might be asked, at what point would the one species 

 end and the other begin. A moment's consideration would show 

 that supposing our knowledge of the evolution of a group of 

 organisms were absolute, we should have a number of species in 

 various ramifying series, each merging imperceptibly backwards into 

 its precursor and forwards into its successor. A further examination 

 would show that the forms designated ' species ' were groups 

 of individuals taken at arbitrary points in the evolutionary series. 

 For these points would not have been fixed upon according to any 

 predetermined plan, as, for instance, to mark a period of comparative 



