8 



year, a somewhat larger butterfly, of a dark violaceous blue colour 

 with plentiful black markings, two conspicuous white spots near the 

 apex of the forewings, and a deep red band running parallel with 

 the margin of both fore- and hindwing. The name given to the 

 insect just described is Preris amestris, the slightly different form 

 found in the southern and eastern regions of Africa being known as 

 P. sesainus. Looking at a specimen of P. natctlensis side by side 

 with one of P. sesaunis from the same locality, we should be inclined 

 to say that they were as different in aspect as two butterflies very 

 well could be ; and when Mr. Trimen published his famous book on 

 South African butterflies (1887) he had no hesitation in considering 

 them as entirely distinct from one another. But there were known 

 at that time a few specimens which seemed to combine the charac- 

 ters of the two species in various proportions ; one of these was 

 figured by Mr. Trimen in his book. The existence of such transi- 

 tional specimens was by him attributed to hybridisation between 

 the two species. 



It had, however, occurred to more than one naturalist that a 

 different explanation of these intermediate forms was possible, and 

 in the year 1896 Mr. G. A. K. Marshall made the definite suggestion 

 that several seemingly distinct species of the genus Precis, including 

 those now under discussion, could be ranked in pairs ; each pair 

 constituting a separate species existing in a two-fold form, one 

 phase belonging to the wet and the other to the dry season. Nata- 

 lensis and sesamus were thus pronounced to be seasonal phases of the 

 satiie species ; and two years later Mr. Marshall was able to supply 

 an absolute proof of the truth of his suggestion, by breeding speci- 

 mens of the dark blue sesamus from eggs laid by the sahiion-coloured 

 7iatalensis. This case is as striking in its way as that of the Euro- 

 pean Araschnia levana and A. prorsa. It was not long before Mr. 

 Marshall obtained equally decisive proof of the correctness of his 

 supposition m the case of another member of the same genus, for 

 he succeeded in breeding two specimens of Precis antilope from eggs 

 laid by the butterfly known as Precis simia. And during the next 

 three years a similar relation was proved by breeding to exist 

 between two more pairs of the genus Precis. Although the same 

 indisputable proof has not yet been forthcoming in every other 

 kind of Precis, there is now no reasonable doubt that Mr. Marshall's 

 original suggestion was in accordance with the facts, and will in 

 course of time be fully verified. 



Many more instances of the same kind are now known to natu- 

 ralists, but before considering any of these we may turn our atten- 

 tion for a time to the question of the meaning of these remarkable 

 alternations in the life, not of the individual, but of the species itself. 

 Ever since the promulgation by Darwin and Wallace of the theory 

 of natural selection, most students of nature have trained themselves 

 to look for some purpose in the outstanding features of all living 

 beings. The search is not always successful, but in a very large 



