70 'WHAT IS A SPECIES?' 



ally, regarded as sufficient. Some of the chief of these 

 categories are briefly set forth under the five heads a-e. 



a. Dimorphism, Polymorphism : — In an ever increasing 

 number of examples an assemblage of individuals is re- 

 garded as a single species, although split up into two or 

 more widely different and sharply separated groups, 

 between which transitional varieties are excessively rare 

 or even unknown. For instance, the extremely abundant, 

 widely distributed butterfly Limnas chrysippus includes 

 among other forms one in which the black-and-white tip 

 is wanting from the fore-wing, the dorippus ( = klugii) 

 form. This variety is sharply cut off from the type form. 

 Although faint traces of a former white bar can be made 

 out in dorippus, I have never seen, among thousands of 

 individuals, the material out of which a good transitional 

 series between it and chrysippus could be constructed. 

 In this case the evidence of Syngamy is strong and com- 

 plete ; for Col. Yerbury has recorded the fact that the 

 two forms certainly occur in copula 1 . But even if this 

 record were wanting there would still be strong presump- 

 tive evidence that the forms are associated by Syngamy 

 and Epigony. Thus, so far as our knowledge extends, 

 dorippus occurs as the only form in certain parts of 

 NE. Africa alone. From this, its metropolis, dorippus 

 spreads on all sides, its individuals existing intermingled 

 with those of chrysippus, becoming less and less numerous 

 until they finally die out. Thus if we trace the two 

 forms eastward we find them both abundant at Aden ; 

 further east, at Karachi, dorippus is well known, but very 

 scarce as compared with chrysippus ; in Southern India 

 it is a great rarity, if indeed it is known at all on 

 the mainland ; in Ceylon a single specimen was cap- 

 tured by Col. Yerbury in 1891, and since then others 

 have been taken. 2 Further east I have never heard 



1 Speaking of his experience at Aden, Col. Yerbury says: 'I have 

 taken them [the forms of chrysippus] " in coitu " in every possible combina- 

 tion' (/oum. Bomb. Nat. Hist. Soc, vii (1892), p. 209). 



2 See Major N. Manders, F.Z.S., in Journ. Bomb. Nat. Hist. Soc, xiv 

 (1902), p. 716:— 



'The first specimen of this insect \dorippus—klugii] in Ceylon was 

 captured by Lieut.-Colonel Yerbury at Trincomalie, April 15, 1891 . . .' 



