xxxii. 4 INCREASING COMPLEXITY OF VERTEBRATES 769 



been going on ever since, to produce the modern amphibia, reptiles, 

 birds, and mammals, inhabiting a great variety of situations. 



In the water tetrapods are found at all levels, including great depths 

 and in perpetually dark caverns. They live in the most varied situa- 

 tions on the land and also by burrowing beneath its surface. Not a few 

 are able to move in the air, some even to feed there. It is hardly possible 

 to overestimate the great variety of vertebrate life; at each new ex- 

 amination of any phase one is amazed at the extraordinary number 

 of special modes of life that are adopted by variants of each type. 



There are no sure means of telling the number of types or of indi- 

 viduals constituting the biomass that was present in past times, but it 

 is probable that by means of the above special devices the vertebrate 

 stock has increased and colonized new regions, though not perhaps 

 continuously or at a uniform rate. It is not unlikely that today there 

 are more and more varied vertebrates than at any previous period. It 

 has been pointed out that the number of species described from 

 deposits tends to increase geometrically with time (Caillaux, 1950). 

 This is not an artefact due to poor preservation. 



Moreover, as Lotka has pointed out, the total energy flux through 

 the system has probably also been enlarged. It would not be easy to 

 demonstrate these conclusions rigorously with our present knowledge : 

 it is difficult to believe that they are true of all populations. We should 

 return from these speculations to reconsider the nature of the evidence 

 about evolutionary change, to discover the changes that we are sure 

 have taken place since the vertebrate organization first appeared. 



4. The variety of evidence of evolutionary change 



At various points throughout the book attention has been called to 

 the conclusions that the evidence allows us to draw, and it is important 

 to notice that they vary considerably from group to group within the 

 chordates. For example, we can draw from morphology some con- 

 clusions about the changes that produced the original fish-like verte- 

 brate, but these conclusions are unsupported by fossil evidence. The 

 fossils available for study of evolution of the earliest gnathostomes are 

 too few to allow us a detailed view of the change from jawless ostraco- 

 derm to placoderms with jaws, and from these to more modern fish. 

 At the other end of the scale, there are so many fossil elephants' teeth 

 to be studied that only an obscure picture of parallel lines of evolution 

 has emerged. Again, in some groups, for instance birds, palaeontology 

 is only of limited help in studying evolutionary change, but nevertheless 

 we have a considerable knowledge about the course of evolution from 



