12 THE VARIATION OF ANIMALS IN NATURE 



The latter meaning is the one used in this work. The pheno- 

 mena to which it is applied are best exemplified by the mimetic 

 phases of certain Lepidoptera. Rarely, seasonal variation 

 may also be found to produce a polymorphic series, e.g. in 

 Daphnia acutirostris Woltereck (1928) found an unusual cycle 

 consisting of spring, summer and winter forms. 



Over and above the variation just described a population 

 may contain other special elements such as castes (e.g. in 

 Hymenoptera), ' high ' and ' low ' males (Scarabaeidae) and 

 developmental phases. 



General theories of evolution have usually concerned 

 themselves with questions as to the origin and importance of 

 new characters and the processes by which the continuous 

 transformation of such characters is brought about. The 

 reference to group-formation in the previous paragraphs 

 stresses an aspect and a result of the evolutionary process 

 which, though they are universally recognised, are perhaps too 

 little regarded. Darwin has been taxed for naming his most 

 important work ' The Origin of Species.' We may admit that 

 he thus gave undue prominence to the species as opposed to 

 other systematic categories ; but the implication that the 

 problem of evolution is closely bound up with that of the origin 

 of groups shows that he realised what to our minds constitutes 

 one of the essential problems of evolution. The formation 

 of groups having some degree of distinctness seems to be a 

 universal property of living organisms, and the whole scheme 

 of animate nature reveals itself as a hierarchy of groups begin- 

 ning with simple aggregates of the status of the pure line, the 

 clone and the colony and developing in distinctness and indi- 

 viduality through the local race to the species and higher 

 categories. 



The main qualitative changes in evolution no doubt begin 

 with changes in single characters, and for the essential features 

 of the process, the linear changes in the history of organs and 

 of one individual type into another, the occurrence of groups 

 is perhaps at first sight irrelevant. As long as the necessary 

 changes occur, the question as to whether they occur in one 

 or 1,000 individuals might seem unimportant. But evolution 

 does not proceed by the transformation of single organisms, 

 but by the mass changes of populations. The outstanding 

 feature of the process as it is seen in palaeontological and 



