sil 
is shared not only (as we have seen) by Phrissura, but also by 
members of several other distinct genera; but all these are African. 
Another very recognisable type of pattern is common to several 
species of Danainæ and the females of many species of Vepheronia; 
this type is found in the Oriental and Australian Provinces, but 
nowhere else on the globe. It is quite true that we come upon 
occasional instances of the occurrence of types resembling some 
of these local developments of pattern in far-removed regions of 
the earth’s surface; but such case are very rare, and in most 
instances may in all probability be fairly put down as accidental. 
There is, for example, a curious little South American Nympha- 
line, Cybdelis mnasvlus, which looks very much like a miniature 
version of the Indian //ypolimnas bolina. But the hardiest framer 
of theories would scarcely venture to suggest any special bionomic 
significance in a phenomenon of this sort. It may legitimately be 
set down as a coincidence. The case, however, is widely different 
when we contrast with sporadic occurrences such as this the 
enormous number of instances in which the forms that so closely 
resemble each other inhabit the same localities, the extensive 
« homœæochromatic » combinations all changing together as one 
passes from one part to another of the same continent; and also 
when we consider the wide prevalence, throughout a given region, 
of a characteristic pattern like the dark ground-colour with a 
crimson band of Central and South America. The facts are unde- 
niable; their interpretation may be in doubt, but to deny that 
there can be any underlying principle to regulate such phenomena 
as these would argue a scepticism so extreme as to pass the proper 
limits of scientific method. 
Now let us turn to a fresh series of considerations. We have 
already noticed the fact of sexual dimorphism in its relation to the 
phenomena of mimicry. There is another kind of dimorphism, 
examples of which are not unknown among the Butterflies of tem- 
perate regions, though its full development must be sought in the 
tropics. I refer to the changes which are observed in successive 
generations of the same Insect in correspondence with the change 
of season, from hot to cold or from dry to wet. An instance of this 
seasonal dimorphism probably well known to all is furnished by 
the European Araschnia prorsa-levana, the spring and summer 
emergences of which Butterfly differ so completely in aspect that 
it seems at first sight impossible to believe that they can be conspe- 
cific. Equally strange instances abound in the tropics, and their 
number has within recent years been increased by the researches 
