D.— ZOOLOGY 97 



and falcate forewings. If there is no corroborative evidence in the 

 specimens themselves, the subspecies based on slight differences in the 

 shade of colour or in the size, and especially the subspecies which overlap 

 each other in size and colouring, urgently require testing by controlled 

 breeding experiments. That such differences are inheritable has to be 

 proved ; the systematist assumes they are, but he may be wrong. The 

 differences between geographical races, however, are frequently very 

 considerable. 



In our researches on the Swallowtail butterflies we came across a 

 combination of distinctions which is most instructive in an inquiry how the 

 subspecies have come into existence. In a large number of species of 

 butterflies and moths the geographical forms are separated by differences 

 in the structure of the organs of reproduction and in colour and pattern , 

 The important point is this, that the two sets of differences vary in- 

 dependently of each other within each subspecies. In Papilio euchenor,^ 

 for instance, the yellow markings of the forewing are less extended, and 

 the hook on the inside of the clasper is less curved in the New Guinea 

 subspecies than in the one from the Bismarck Islands. If a New Guinean 

 specimen somewhat approaches the Bismarckian race in colour it does not 

 show an approach in the shape of the hook of the clasper, and vice versa. 

 The chance that a specimen of one race approaches the other both in 

 colour and structure is very remote — we have never come across one — ■ 

 and the identical combinations colour plus structure of the Bismarckian 

 race cannot be expected ever to occur among the New Guinean popu- 

 lation. Therefore the Bismarckian subspecies cannot have come into 

 existence by arrivals from Guinea, having already possessed the charac- 

 teristics which distinguish the race of the Bismarck Islands ; con- 

 sequently these special distinctions must have been acquired after the 

 islands had become populated from New Guinea, no matter whether 

 the immigrants were average or not. The individual characters of the 

 ancestral specimens do not influence the formation of the new race, only 

 what is inheritable is of importance, and what is non-pathological and 

 therefore adaptable to new and possibly less congenial surroundings. 

 It is perhaps necessary to emphasise that the breaking-up of a species 

 into geographical races (subspecies), often into a large number, is not 

 exceptional, but is the rule with all species with wider distribution, and 

 that the above combination of structure and colour has been tested in 

 many species. A chain of races each confined to its district is a beautiful 

 illustration of the workings of evolution. The differences evolved 

 during isolation depend on the constitution of the animal and the nature 

 of the environment, and the change may be visible only in externals, 

 or may affect also internal organs. In mammals the subspecific characters 

 relate generally to the skull and the colour, texture and proportions of the 

 skin ; in birds to wing-length, proportions of the bill and to colour ; in 

 insects, where the whole skeleton and the soft parts, at least in a dried-up 

 state, are preserved, distinctions may be found in any part of the body, 

 but, apart from colour and pattern, are often most pronounced in secondary 



* Cf. Roths., Nov. Zool., vol. ii, p. 339 (1895) ; Jord., I.e., vol. iii, p. 469 

 (1896). 



