CAEADOCIAN CYSTIDEA FROM GIRVAN. 511 



modifications depend on the combination of a stationary microphagous existence with 

 a position different from that of normal erect Pelmatozoa. 



§ 613. The normal pelmatozoic habit is best adapted for deep, calm waters, and the 

 connection of these departures from it with shallow-water conditions has been well 

 brought out by Dr Kirk. In these early examples the modifications were more pro- 

 found and far-reaching than in later times, simply because the forms affected had not 

 yet acquired all the characters of the normally specialised Pelmatozoon. They were 

 able to adapt themselves more readily to the numerous possibilities afforded by a varied 

 and variable habitat. Hence their extreme specialisation in diverse directions. 



§ 614. Finally, the question has been put : if these animals were each and all so 

 admirably adapted to their surroundings, how was it that they all became extinct 

 directly afterwards ? Not one of these genera, so far as we know, survived to Silurian 

 times. Not one of the Silurian genera that took their place can be regarded as a direct 

 modification of any one of them. The answer seems to lie in the change of conditions 

 that must have passed over the whole North Atlantic region at the close of the 

 Ordovician Epoch, and that left its impress on the whole of the marine fauna. The 

 conflict between inherited structure and altered surroundings commenced anew ; but 

 by this time the hereditary tendencies had become so fixed that they could not yield 

 rapidly enough to the needs of adaptation. Had these creatures really been free 

 wanderers they might have escaped destruction by migrating to their accustomed 

 surroundings elsewhere. But, so far as we know, they did not do so ; and even if they 

 had made any unconscious attempt, their way would doubtless soon have been blocked 

 by some ridge of land or some deeper trough over which they could not pass. Thus, 

 when somewhat similar conditions reappear in Silurian rocks, the Echinoderms modified 

 to take advantage thereof have been derived from other Families. 



So does the remote story of these Girvan fossils reveal anew the perpetual warfare 

 of life, its ever unaccomplished task to bring this ancestral burden of the body into 

 perfect harmony with an elusive nature ; for, just as that highest hope falls to be 

 fulfilled, the wheel of the world is turned, and the new contest demands competitors of 

 a less wearied stock or a more flexible training. The Palaeontologist may be one who 

 deals with creatures that are what we call " dead and buried," but he above all men should 

 be filled with a sense of the living drama that underlies his science : the unending 

 struggle of the past as crystallised in organic form with the ever new surroundings of 

 a shifting universe ; the clash of inherited structure, habit, thought, against the iron 

 necessities of the present. The figured stone, rudely cast aside by the quarryman or 

 blindly treasured by the curio-hunter, is for him an emblem of shattered conventions 

 and creeds outworn ; it is his material proof of the eternal Nemesis. 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 67 



