( 521 ) 



respect, to propagatiou compared with the uormal specimeus iu whii'h cDpnhitiou is 

 more facilitated ; monstrosities and sports iu respect to the genital armature, in 

 short every specimen with an anomalous development of the organs of copulation 

 is less favoured than the rest of the individuals. Such abnormal individuals have, 

 therefore, the same position in the struggle for propagation, as in the struggle for 

 existence those specimens have which are less adaj)ted to the circumstances of life 

 than their cospecific rivals. If it is right to conclude in the latter case that the 

 struggle for existence leads iu the long run to a survival of the fit {Romanes), we 

 can ajiplv the same inference to our special question and say that the specimeus 

 abnormal iu the genital organs will in the struggle for propagation succumb, and 

 that consequently the anomalies iu the organs of copulation will become rarer 

 and rarer, the individuals acquiring gradually by breeding iu and in the same 

 form of prehensile organs, comparatively few specimens deviating widely from 

 the normal. It is apparent that the mechanical selection thus eifected will 

 end in a comparatively great constancy of the preliensile apparatus.* A number 

 of the forms of Papilio dealt with iu this- paper have attained a remarkably high 

 degree of constancy in the genital armature, for instance several forms of P. sarpedoii 

 and P. aristeus, P. alcinous confusus. As the conclusion applies to every segregate 

 form of Papilio, it is evident that, when a species develops divergently into 

 geographical races, each race (or some of them) can, and will, acquire a special 

 normal form of the genital armature, if the evolutionistic factors of the locality 

 affect the organs of copulation. Many species have not yet attained that degree of 

 divergent ramification. The geographical forms of Papilio aegeus, machaon, some' 

 of the races of P. sarpedoii, namely sarpedon sarpedon and sarpedon semij'asciatiis, 

 the two eastern races of P. aristeus, P. aristeus aristeus and P. aristeus panmitus, 

 and others, we have found to be identical or almost so with one another iu the 

 genital armature. In the geographical forms of other species the specialisation in 

 the prehensile organs is already visible, but not very conspicuous, as in P. rlounthu.-< 

 chjmenus and P. cloanthus cloantkus ; while in others again the degree of divergency 

 in the organs of copulation of geographical races is so high that the normal of one 

 form differs from the normal of the other so much that the one stands far outside 

 the usual limits of variation of the other. Now, when it happens that forms so 

 diverse in the genital armature inhabit neighbouring districts, as is the case with P. 

 sarpedon teredon (S. India), /'. sarpedon sarpedon (N. India to the Philipjiines and 

 Java), and P. sarpedon milon (Celebes) ; P. alcinous confusus (China and Formosa) 

 and P. alcinous loochooanus (Loo Choo Islands) ; P. bathycles chiron (N. India to 

 Burma) and P. hathijcles bathi/cloides (Malacca), it is manifest that the occasional 

 (passive or active) migration of specimens from one district to the other, which we 

 must concede to occur, as otherwise the wide range of many species would not be 

 explainable, can have no great retarding effect on the divergent development of the 

 forms, since an effective copulation of the immigrants with the occupants of the 

 district is improbable, and as further, if copulation should occasionally be effective, 

 the characters of the descendants of the new-comers will soon get swamped awa}' 

 by mechanical selection. Thus it is conceivable that, when a certain degree of 

 divergency is attained by geographical races, mechanical selection will greatly help 

 to accelerate evolution, and I hat it is able to prevent retrograde development when 

 the geographical barrier which formerly separated two forms has, in consequence of 



* Marked dimorphism in the organs of copulation to wliicii tlie variation ol' the organs could also lead 

 does not seem to occur ; see p. 499. 



35 



