( XXXV ) 



our arj;-umeiit than any words. The diagram ilhistratcs an extreme hypothetical 

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

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

 sentatives, but the means are different. The small number of specimens, a d, 

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

 the small proportion b c oi the specimens of A^ are tlie 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 e 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, thongii 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 shajie ; one form 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 



di- or polymorjihic, it is necessary to comj)are 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 

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

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

 The 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 t3'pe of 

 animal, inhabiting, for instance, each one particular island, and W, B-, B^, B^, etc., 

 those of another type of animal found on the same islands respectively, 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 tlie question arises. Are the gaps between tiie various A's 



* The ordinate gives the number of individuals, the abseissa the degree of difference. 



