364 THE PLACE or MIMICRY 



of species. Tlie lar^;e Danaine butterfly, Anosia plcxippus 

 (archippus) alhuled to on p. 274, has now travelled from its 

 home in the New World to man\- parts of the Old, but the 

 eftect it has i^roduced upon the butteril)- fauna of North 

 America would always show, even if we had no further 

 evidence, the position of its ancestral home.' So, too, 

 the commonest Panainc in the Old World, Linnias 

 clirysippiis, althoui^h ranij;in^ through and beyond the 

 Oriental Region, is clcarh shown by the same kind of 

 evidence to have been an inhabitant of Africa for a far 

 longer period. Not only is this inference justified, but 

 it can also be stated that the exclusive prc-clominance of 

 the white-hind-wing^ed alcippus form of chrysippns on the 

 west coast of Africa, from the Ni<^er Mouths to the 

 Sahara (see p. 321), is a result of modern changes in 

 distribution. It has not affected any of the West Coast 

 mimics. excei)t perhai)s Acraca cnctdon, in which case 

 there is some evidence, from one locality in Sierra Leone, 

 to show that the jjresence of a/cippns has encouraged the 

 white-hind-winged^rA"///^^^^^ form.- Furthermore, a study 

 of mimetic forms may enable us to reconstruct the past 

 stages through which the older model has passed. 

 When a o-eoloo^ist finds a recognizable fratrment of 

 one rock included in a stratum of another, he infers 

 with confidence that the latter is the younger. With 

 e(jual certainty the zoologist may conclude that the 

 mimicking species is younger than the model it has come 

 to resemble. In certain cases the species acting as a 

 model is di- or trimorphic. Thus the individuals of 

 Livnias cJirysippus fiill into three groups, of which two, 

 as described on pp. 70, 71, are sharply marked off and 

 unconnected by gradual transition. The individuals of 

 the two commonest mimics of cJirysippus also fall into 

 three corresponding groups. In both species of mimic, 

 however, all the grou[)S are connected by abundant 

 transitional forms. We are led to believe that the 

 mimics, being younger, present us with a picture of the 

 former condition of the older model, when transitional 



' Proc. Af/itr. Ass. Adv. Sci., vol. xlvi, 1897, p. 244. 

 ^ Trans. Etil. Soc, Lone/., 1902, p. 480. 



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