THE EVOLUTIONARY CAREER 309 



in the light of our knowledge of the structure of present-day 

 animals, the hard skeletons, and perhaps other parts which can 

 be compared with the fossil structures. 



Such a representation of a phylogeny, based on the occurrences 

 of fossil remains and of their structural similarities and differences, 

 suggests the system of branchings of the twigs and main branches 

 of a tree : the trunk of the latter representing some hypothetical 

 common stock from which we infer that the various groups of 

 organisms, represented by fossil remains, have diverged, or 

 evolved. Actually, however, the picture that paleontology 

 suggests is that of a system of divergent branches that do not 

 actually meet in a trunk, but which converge to a trunk. 



103. ON THE MAIN FEATURES OF THE EVOLUTIONARY 



CAREER 



These are diagrammatically represented by figures which have 

 been constructed on the principles indicated above. We see, 

 then, that both in respect of animals and plants (Figs. 41-4) a 

 number of main types of organisms appear to come into origin 

 independent of each other at about the beginning of the Paleozoic 

 periods (in the cases of the animals) and — with the exception of 

 the Thallophytes — a little later in the cases of the plants. 

 These main types of organisms, having once appeared, continue 

 to maintain themselves up to the present times. In the history 

 of each main type there have been '' episodes " (Section 106) 

 w^hen " embroideries on the types " have appeared, have flourished 

 and have become completely extinct, or have left only some 

 unimportant modern representatives. 



It would be WTong to suppose that the appearances of these 

 main types were actually independent origins of living things, as 

 the diagrams suggest. Such an interpretation is improbable 

 in view of the great periods of time (Proterozoic and Archeozoic) 

 which undoubtedly were such as to permit of life, but which are 

 not represented by fossils capable of structural description. We 

 must assume that there was a long pre- Cambrian period 

 throughout which these main types of life were being evolved and 

 when, it may have been, they did evolve from one original life 

 stock. Still the possibility of the independent evolutions of 

 various modes of metabolism must be borne in mind and, as 

 structural characters are really made by these modes of functional 



