CHAPTER II 



Processes of Evolution^ 



Evidence of the fact of evolution can be found in most fields of 

 biological study. But facts which give some indication of the genetic 

 mechanisms involved are much fewer. Most of them fall into one of 

 two classes, which demonstrate two important theses. Firstly, those 

 fossils whose evolution can be followed in a continuous series through 

 some considerable period of time give conclusive evidence that evolu- 

 tionary change can be by gradual transitions, which, moreover, progress 

 in a single direction. Secondly, there is evidence, perhaps not quite so 

 strong, but nevertheless very cogent, that some evolutionary changes 

 can be by sudden and discontinuous jumps. This has emerged most 

 clearly from studies on the distribution of plants. 



I . The Palaeontological Evidence^ 



Palaeontology provides evidence about the evolution of the hard 

 parts (skeleton, shell, etc.) of organisms which were common enough 

 to be frequently preserved as fossils. For the theory of evolution there 

 are two fundamentally important classes of evidence, which concern 

 quite different time-scales and which must therefore be sharply dis- 

 tinguished. On the one hand, we can describe the general outUne of the 

 course of evolutionary change in a large group of organisms, in some 

 cases even in a whole phylum. We are then dealing with phenomena 

 which last throughout many geological periods, probably through 

 times of the order of some hundreds of millions of years. On the other 

 hand, we have data as to the gradual alteration of one species or small 

 group of species, a process which usually takes place within one of the 

 more recent geological periods, over a period of a few million years. In 

 both cases students of variation among living animals will be surprised 

 by the high degree of orderliness which palaeontogists ascribe to the 

 processes concerned. On the large scale, whole orders and phyla seem 

 to pass through an orderly series of changes which have been described 

 as "programme evolution"; while on the small scale it has been shown 

 that a group of "species" may evolve progressively along a certain line 



^ General references: Haldane 19326. 



^ General references: Bather 1927, Davies 1937, Lang 1923, Swinnerton 

 1923, 1932. 



