8i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



is indicated by the popular phrase "The Ascent of Life", or "The 

 Advance of Life", that in the course of the geologic ages there has 

 been a gradual emergence of more and more highly differentiated 

 and integrated organisms. The "geological record", albeit far more 

 than half-concealed in the fossil-bearing rocks, shows that there 

 was an Invertebrate fauna for long ages before there were any 

 Vertebrates, that Fishes thronged the seas long before the pioneering 

 Amphibians began to struggle on to dry land towards the end of the 

 Devonian Period. For a prolonged period, from the Permian onwards, 

 the highest backboned animals on the earth were Reptiles, of all sizes 

 and sorts, and each expressing in their own way varied possibilities 

 of locomotion — first creeping and then running on the solid ground; 

 swimming in pool and river; and even thence returning to the sea, 

 or parachuting and even flying in the air. Their alternatives doubtless 

 descended to burrowing, and rose to tree-climbing as well, as a good 

 many living forms of reptiles still do. But of burrowing and tree- 

 climbing reptiles there are no fossil remains. Only when the Reptiles 

 were passing their Golden Age do we find sparse representation of 

 primitive Birds and pigmy Mammals. The large fact is that for 

 mHlions of years there was a growing dynasty of Reptiles, with as 

 many modes of life, or "adaptive radiations", as we see in the 

 Mammals of to-day. 



What is true of the great classes of Vertebrates, that the higher 

 ones were latest to appear, holds true of the orders within these 

 classes. Thus we find simple and primitive mammals long before 

 the appearance of specialised orders, such as Cetaceans ; while of the 

 extremely differentiated bat-tribe there are few fossils and none 

 primitive. Similarly within particular orders; thus the earliest 

 representatives of the elephants, or of the horses, were much more 

 generalised than those that succeeded them, or are surviving to-day. 

 As ages passed there was, on the whole, a gradual increase in 

 differentiation and integration at most levels of life, except where 

 racial senescence or retrogression set in. 



Certain as is the Ascent of Life from age to age, three saving 

 clauses must be noted, {a) In the earliest period with abundant 

 fossils, that known as Cambrian, most of the great groups or phyla 

 of Invertebrates are already represented. This at first sight surprising 

 fact becomes intelligible when we recognise that there had been a 

 very long history before the Cambrian— a history of which there 

 are no records at all in the Archaeozoic rocks, and not more than 

 a few, and these very fragmentary and puzzling, in what we thence 

 caU the Proterozoic. In short, there must have been a prolonged 

 Pre-Cambrian evolution, both for plants and animals. 



(b) In the second place it must be recognised that amid the 

 general advance there have been particular retrogressions. Evolution 

 may anj^where proceed downwards as well as upwards; and retro- 



