108 EVOLUTION OF LIVING ORGANISMS. 



the transformation of Planorbis through the successive 

 geological strata of Steinheim. 



In this record of the past we read the work of natural 

 selection, the drastic action of elimination, and see on a 

 large scale what is happening to-day not only among the 

 competing groups of organisms, but among the struggling 

 individuals. From the record we also learn that evolu- 

 tion does not proceed along an even course such as we 

 might expect to see pursued owing to the pressure of 

 some internal or external directive force. On the con- 

 trary, it is the rule that groups quickly expand, radiat- 

 ing in various directions of adaptation. This specialisa- 

 tion leads to a certain rigidity, a loss of adaptability in 

 other directions, and sooner or later to a failure to meet 

 new conditions, while some obscure side branch com- 

 mitted as yet to no special line of adaptation acquires 

 some advantageous combination of characters, enabling 

 it to compete successfully with the dominant race. 

 Evolution does not move along a straight line from one 

 dominant specialised form to another, but by the con- 

 stant uprising of new forms which supplant the old ones. 

 The evidence of Palaeontology is all against the theory 

 of orthogenesis (the transformation of one group into 

 another along a straight line in ladder-like fashion). 

 Adaptive radiation, a perpetual tendency to branch off 

 in various directions, and founded on individual varia- 

 tions of indeterminate character, is seen in the history of 

 all groups of organisms. The number of possible lines 

 of development is indefinitely great ; the external en- 

 vironment decides which, if any, shall succeed. 



So it is not from the specialised Amphibia that the 

 reptiles have been developed, but from some early un- 

 differentiated form ; neither is it from specialised Kep- 

 tilia that the Mammalia have been derived. And among 

 the mammals themselves, the Carnivora, Uugulata, 

 Cheiroptera (bats), the Cetacea (whales), have not de- 

 scended the one from the other ; but all have diverged 



