DE. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 569 



regard those sub-typical examples of 0. complanata, which exhibit the transition I 

 have described from the " simple " to the "complex " plan of structure, not as advanced 

 forms of either of the two " simple " species, but as retarded forms of the highest ; and 

 a clue to the conditions of that retardation can, I think, be found in the marked 

 inferiority I have invariably observed in the size of the original nuclear mass of these 

 individuals. I drew attention in my former Memoir ( 44) to the remarkable range 

 of dimension which this mass exhibits, when a considerable number of specimens are 

 examined ; and showed that the cavity of the " primordial " and " circumambient " 

 chambers in one individual must have been more than a hundred times as large as 

 that of another. Now the result of the far more extended comparison of specimens 

 which the " Challenger " collection has enabled me to make, is, that while the " nucleus " 

 of the typical 0. complanata (in which the cyclical plan of growth, and the "complex" 

 structure, show themselves from the very first) is always many times larger than that 

 of either 0. marginalia or 0. duplex, the " nucleus " of its sub-typical forms always 

 bears a very close accordance in size to that of 0. duplex, which it resembles also in 

 the one-sided pullulation of the first sub-segments from the circumambient segment, 

 rendering the earlier zones more or less incomplete, and the position of the " nucleus " 

 slightly eccentric. Whether these forms genetically propagate themselves as a race, 

 perpetuating an earlier stage of the evolution of the perfected type, or are merely 

 individuals which have begun life as " starvelings " that do not inherit the characteristic 

 vigour of the type, I have no adequate ground for even surmising ; being only able to 

 affirm this, that as there is no kind of constancy in the stage of growth at which the 

 " simple " plan gives place to the " complex," there is nothing to justify a specific differ- 

 entiation of this sub-typical variety. That its peculiarity may depend upon conditions 

 less favourable to the full development of the type, seems to be indicated by the fact 

 that, whilst the largest and most typical specimens of 0. complanata were found in the 

 rock-pools on the summit of the Fiji reef, where they would have the highest tempera- 

 ture and the greatest abundance of food, the sub-typical specimens presented themselves 

 chiefly in the collection made by the dredge at 18 fathoms' depth. 



Tlieory of Descent. 



I propose, in the last place, briefly to examine the bearing of the remarkable case 

 of " descent with modification," which I have thus detailed, upon the general 

 " Theory of Descent " and of the " Origin of Species." 



Those who find in " natural selection " or the " survival of the fittest " an 

 all-sufficient explanation of the " origin of species," seem to have entirely forgotten 

 that before " natural selection " can operate, there must be a range of varietal forms 

 to select from ; and that the fundamental question is (as Mr. DARWIN himself clearly 

 saw, at any rate in his later years), what gives rise to variations ? No exercise of 

 " natural selection " could produce the successive changes presented in the evolu,- 



