PHYLETIC SKRIKS 361 



Trigonia among the Lamellibranchs, each comprise perhaps 

 more than twenty independent phyletic series" (Deperet, 

 p. 200). 



Variation along the phyletic lines is gradual l and 

 determinate, and appears to obey definite laws. The 

 earliest members of a phyletic series are usually small in size 

 and undifferentiated in structure, while the later members 

 show a progressive increase in size and complexity. Rapid 

 extinction often supervenes soon after the line has reached 

 the maximum of its differentiation. 



The general picture which palaeontology gives us of the 

 evolution of the animal kingdom is accordingly that of an 

 immense number of phyletic lines which evolve parallel to 

 one another, and without coalescing, throughout longer or 

 shorter periods of geological times. " Each of these lines 

 culminates sooner or later in mutations of great size and 

 highly specialised characters, which become extinct and leave 

 no descendants. When one line disappears by extinction 

 it hands the torch, so to speak, to another line which has 

 hitherto evolved more slowly, and this line in its turn 

 traverses the phases of maturity and old age which lead it 

 inevitably to its doom. The species and genera of the present 

 day belong to lines that have not reached the senile phase ; 

 but it may be surmised that some of them, e.g. elephants, 

 whales, and ostriches, are approaching this final phase of 

 their existence " (Deperet, p. 249). 



It is one of the paradoxes of biological history that the 

 palaeontologists have always laid more stress upon the 

 functional side of living things than the morphologists, and 

 have, as a consequence, shown much more sympathy for 

 the Lamarckian theory of evolution. The American palaeon- 

 tologists in particular Cope, Hyatt, Ryder, Ball, Packard, 

 Osborn have worked out a complete neo-Lamarckian theory 

 based upon the fossil record. 



The functional point of view was well to the fore in the 

 works of those great palaeontologists, L, Riitimeyer (1825- 

 1895) and V. O. Kowalevsky (1842-83), who seem to have 

 carried on the splendid tradition of Cuvier. Speaking of 



1 The steps in this chronological variation were termed by Waagen 

 " mutations." 



