140 The Classification of Insects 



differs from astrarche : in the simpler pattern of its 

 lower wing-surface — plain white spots instead of 

 black-centred eye-spots. It is probable, therefore, 

 that astrarche is in reality the newer form, and that 

 the Scottish artaxerxes does not represent a special 

 development due to climatic conditions, but a survival 

 of an old stage in evolution passed by elsewhere — 

 comparable to the Gaelic speech of the people who 

 share its special homes. It will be remembered also 

 that this same variation — the disappearance of the 

 black centres of the eye-spots — sometimes occurs in 

 P. kariis, the other (blue) butterfly mentioned in 

 connection with P. astrarche. If the explanation just 

 given be correct, this would be an instance of the 

 occasional reappearance of an ancestral character. 

 Such variations, which recall a past stage in the 

 history of the race, are not uncommon in various 

 animals. They are defined as atnvic variations. 



The Origin of Species. — We have seen that 

 it is impossible strictly to distinguish a "species" 

 from a " variety." There is the strongest presumption 

 that the marked and definite distinctions to which 

 the former term is applied have their origin in varietal 

 differences. The problem of the evolution of distinct 

 species from varieties has now to be faced. The 

 difficulty of this problem was felt to be so great that 

 until forty years ago belief in the fixity of species 

 prevailed generally among naturalists, who thought 

 variation must be kept within such strict limits by the 

 action of heredity that species must ever be and must 

 always have been distinct. A necessary consequence 

 of this belief would be that all kinds of animals 

 originated independently of one another by a process 

 which one cannot even try to explain. This view 

 was abandoned by naturalists, and belief in evolution 

 was adopted mainly through the influence of Darwin, 



