BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 99 



as the Olenellus and Paradoxides of the Menevian Beds and 

 Primordial Zone of Wales and Bohemia, we have undoubtedly 

 a vast frame for the picture of organic form and organic change. 



When we look at the bottom of this picture and then at the 

 top we are at once struck by the enormous character of the 

 change revealed. Although there were highly developed 

 trilobites in the Cambrian seas with all the Classes of Mollusca 

 abundantly represented in the Ordovician or Lower Silurian 

 period, yet no vertebrates appear to have existed in any part 

 of those most lengthy epochs. 



The backbone, the basis of the skeleton of the animals 

 which to so large an extent people the earth and its waters 

 now, was then non-existent, its advent being in a long 

 subsequent Upper Silurian period. This remarkable morpho- 

 logical feature, the backbone, with its most important 

 physiological attributes, is undoubtedly the most conspicuous 

 differentiating characteristic of the post-Ordovician fauna. Its 

 appearance gave to the world the fishes of the seas, then the 

 amphibians of the shallow waters, and afterwards the great 

 dinosaurs, the pterosaurs and other Keptilia, to be followed by the 

 marsupials and monotremes of the land and the feathered birds 

 of the air, with, long subsequently, the larger Mammalia unlike 

 to those we now see, to be succeeded by the larger Mammalia 

 in forms akin to those we know as living creatures, and lastly, 

 the speaking and reasoning genus Homo. 



It is this vast development of the Vertebrata in both greater 

 and lesservariation,in those great differences that constitute Class 

 distinctions as well as in the smaller differences of genera and 

 species, together with the great increase of individuals, that 

 alters entirely the upper part of the great picture of life on the 

 globe from the Cambrian times to the present. The appearance 

 of the backbone marked, consequently, a most momentous 

 period in the life-history of our planet, whicii seemed, as it 

 were, to be a fresh starting-point for organic development. 

 The concentration of the nerve-matter of the animal in one 

 cephalic ganglion, the brain, accompanied by an incipient and 

 then by a developed vertebral column and canal, must be 

 regarded as the greatest biological change that the fauna of the 

 globe has undergone, inasmuch as it was the necessary step on 

 the road to all subsequent developments of animal life. 



But while this great development of vertebrate animals was 

 in progress, changes by no means small were taking place in the 

 Invertebrata also. An entire Order of Actinozoa, the Rugosa, 

 disappeared, while three others advanced. Two Orders of 



