( sxxv ) 



our argument tlmn any words. Tlie diagram illustrati.'s an extreme hypothetical 

 case. The curves A' and A- represent the variation * of two geographical 

 representatives. The extent of variation, a r, is the same in both repre- 

 sentatives, bnt the means are different. The small number of specimens, a h, 

 belonging to A^ are identical with the majority of the individuals of A', while 

 the small proportion 3 e of the specimens of A' are the same as the majority 

 of A^. It is obvious that there is little chance of the student getting any of 

 the specimens a b of A- and b c of A', if he has not a really large material 

 at disposal. To him A^ and A- would appear to be separated by a gap ; they 

 would appear to be constantly different, thougli they are, each taken as a whole, 

 morphologically identical. 



Two sexually or otherwise di- and polymorphic animals do not- differ in 

 all specimens in the same way ; one sex or one set of individuals may be 

 distinguishable by colour, the other by shape ; one foi'm may be paler, another 

 darker, than the respective forms of the animal with which they are compared. 

 To find out the differences between two geographical representatives which are 



Fig. 1. 



di- or i)olym(irphic, it is necessary to compare the corresponding forms with 

 one another, just as one has to compare sex with sex, larva with larva of 

 the same stage, pupa with pupa. There are also monomorphic geographical 

 rejiresentatives which differ from one another in a similar way, some individuals 

 differing in shape, others in colour, others again in pattern or in structure. 

 Tlie difference between the two representatives is also often constituted by the 

 ensemble of the characters in eacli individual. 



Such cases lead over to those in which the geographical representatives are 

 completely separated by a gap in their morphology, either in one or in several 

 organs. If A', A^, A^, A^, etc., are the representatives of one certain type of 

 animal, inhabiting, for instance, each one particular island, and B', B", B', B^, etc., 

 those of another type of animal found on the same islands respectivel}', there is 

 a corporeal gap between A' and A^ etc., and between B' and B^, etc., which 

 do not live together, as well as between A' and B', A" and B'', etc., which live 

 together. And now the question arises. Arc the gaps between the various A's 



* Thi; ortliiiate givos llic number of indiviiliuils, the abscissa the degree ot lUll'ercncc. 



