BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 101 



fauna, however, of the Secondary epoch with its huge armed 

 dinosaurs and its flying pterosaurs was markedly different 

 from that of the Tertiary epoch, and therefore a great change 

 was produced by the extinction of tliose reptilian monsters. 



The result of all biological change has, however, been to give 

 to the globe a succession of higher and higher forms with 

 greater complexity of structure and higher physiological power 

 and capabilities. 



Persistency of Types. 



"When we look a little closer at the wondrous picture and 

 examine its details both in its lower and its upper portions, we 

 are struck by the marvellous persistency of certain forms and 

 structures, and the persistency, too, of the functional power and 

 purpose of similar organs. We see forms close to the bottom of 

 the picture and we see similar forms at the top, even the very 

 top. So like do they appear that it requires close scrutiny by 

 trained and expert observers to detect any difference. And 

 w^hen it is borne in mind that of the organisms existing in the 

 far- back Cambrian period only a few can have come to our 

 notice, we must conclude that very many of the lower organisms 

 of the present day are generically related to organisms of the 

 Cambrian period. This compels a recognition of the unity of 

 the whole organic world which must be regarded as one great 

 biological chain without a break and with every link connected 

 with another throughout geological time. 



Although the trilobites which were so abundant in older 

 Palaeozoic times became extinct before the Secondary epoch, yet 

 the Limulus or King Crab of the present day, especially in the 

 young state, strikingly reproduces their main features, and the 

 sessile and compound eyes of the common crayfish, crab, and 

 lobster, are almost identical with those of the Calymeiie and 

 Phacops of Silurian times, in some of which trilobites very 

 many facets in each eye have been counted. The four-eyed 

 Limulus first appears in Jurassic, but the allied Neolimulus is 

 in Upper Silurian strata, and the eurypterids of these Palseozoic 

 rocks are scorpion-like also, and are now regarded as Scorpion- 

 idaB and Arachnida, although aquatic, the present scorpions and 

 spiders differing in being air-breathers, even as land snails differ 

 from aquatic gasteropods, There is, moreover, a true scorpion 

 in Upper Silurian rocks, the Palceophonus Hunieri, while from 

 Carboniferous strata no less than seventy-five species of Arach- 

 nids have been obtained. 



