150 Novit ates Zoologicae XXIII. 1916. 



as are not visible outwardly,* the specific distinguishing characters cannot have 

 developed by means of selection on the part of insectivorous enemies. 



In some of the species we find small but obvious morphological differences in 

 the specimens from certain localities : geographical varieties or subspecies. In 

 other forms which are also geographically separated the differences are so great 

 that we must consider them specific ; these species replace one another, one 

 being a substitute for the other : vicariant species. And lastly, we have species 

 the- ranges of which overlap or are' more or less the same, the species occurring 

 side by side, often actually flying together : synpatric species. This gradation 

 in the evolution is represented, for instance, by (1) Syckesia dry as dryas and 

 S. dryas tupus, which exclude one another geographically, but are essentially 

 the same insect; (2) Ammalo insulata, A. arravaca and A. aurata, which also 

 inhabit separate geographical areas, bat are so different that they might occur 

 together without mixing and amalgamating ; and (3) Syckesia dryas, S. subtilis 

 and S. omissus, which occur together. If the differences in the first category 

 become greater, we have the second. If the range of the second category extends, 

 we have the third category of forms. This is true not only for insects, but also 

 for other classes of animals, the exceptions appearing to me more seeming than 

 real. The important part which geographical isolation plays in the evolution of 

 the subspecies and vicariant species is so obvious that it is hardly necessary to 

 dilate on it. Which, however, is the factor or group of factors that led to the 

 appearance of the structural differences we have described ? Mendelism cannot 

 account for the geographical phenomenon embodied in the problem ; selection by 

 insectivorous enemies being likewise excluded, there remains the influence of the 

 anorganic surroundings, which are different in the various geographical areas, 

 in connection with geographical isolation. If the geology of a continent or 

 archipelago is known, i.e. the relative ages of the districts or islands, one can 

 generally predict with a high degree of accuracy where subspecies and vicariant 

 species will be found. 



I have assumed that the various species of Syckesia, and the three yellow 

 Ammalo, have retained the colouring of the respective ancestors from which they 

 are derived, and for this reason are externally so similar to one another. The 

 opinion, however, might be advanced that these species were originally also 

 different in colouring, and their present synchromatism is a secondary development 

 due to mimicry. This cannot be a true explanation, because (1) most of the 

 species are not synpatric, and (2) the subspecies of S. dryas and S. subtilis are 

 alike in colour and different in structures in a similar way as are the species, 

 but to a much lesser degree. 



