REPORT ON THE GENUS ORBITOLITES. 43 



having been found in certain Jurassic strata ; and the Cretaceous forms to which 

 the generic name CycloUna was given by d'Orbiguy may perhaps be referable to it. But 

 without a careful examination of their internal structure, it cannot be said with any 

 certainty whether these were Orbiculince (as the prominence of their centre would seem 

 to indicate) or true Orhitolites. It seems to have been in the comparatively shallow and 

 probably warm waters of the Maestricht Chalk that the more specialised Orbitoline type 

 first became conspicuous. 



CONCLUDING SUMMARY, WITH A STUDY OF THE THEORY 



OF DESCENT. 



Thus it has been shown, that whilst an examination of the central nucleus of the disk 

 of Orhitolites tenuissima enables us to trace back the pedigree of the Orbitoline type to 

 the very simplest " jelly -sjaeck " that can form a porcellanous shell, an examination of the 

 inner rings of certain disks of the highly specialised Orhitolites complanata makes it clear 

 that this most " complex " of Orhitolites (the most heterogeneous in structure of aU existing 

 " porcellanous " Foraminifera) has had its origin in the most " simple." And yet, as has 

 been also shown, this progressive complication of the calcareovis skeleton does not seem 

 to involve either any corresponding dilFereutiation of parts in the sarcodic body, or any 

 such change in its physiological character as implies a higher or more special adaptation 

 to the conditions under which these animals exist. 



It was sagaciously remarked by Sir James Paget,* long before the Biological revolution 

 wrought by the publication of the Origin of Species, that " the highest laws of our 

 science are expressed in the simplest terms in the lives of the lowest orders of creation." 

 And in accordance with this view, I propose to make this remarkable group of facts the 

 subject of a "Study in the Theory of Descent," for which it presents the following specia 

 advantages : — 



First, that the remoter ancestry, instead of being indicated (as it commonly is in the 

 developmental history of the higher organisms) by obscure and transitory phases, is here 

 distinctly represented in the earlier stages of the completed form. Thus, if the develop- 

 ment of a very young Orhitolites tenuissima were checked in its early Milioline stage, it 

 would be accounted a Spiroloculina ; if checked in its short Peneropline stage, it would be 

 accounted a true Peneroplis ; and if checked in its Orbiculine stage, it would be 

 accounted a true Orhiculina. And so, if the development of the "sub-typical" variety of 

 Orhitolites complanata were checked in its first stage, it would rank as an Orhitolites 



' Lectures on Repair and Reproduction, delivered at tlie Royal College of Surgeons in 1848. 



