Vii 
races of a species are sub-species, is not to be found in their 
segregation, or in the amount of their differences, but in the 
circumstance, that these differences have a permanence under 
any disturbance as of habitat, etc., that makes some approach 
to the permanence under such disturbance of a species. 
That this can rarely be ascertained makes it necessary that 
we should observe a doubtful attitude in most cases, admit- 
ting that we are unable to decide in either way, unless so 
strong a fact as geographical separation obtains.’’ 
For example :— 
1. Mopuza CattpaAsA. This is peculiar to Ceylon, but 
is closely allied to M. procris of India. It is found at all 
elevations, and in both the wet and dry zones, but I believe 
that no varieties approaching M. érocris are known. The 
differences are apparently permanent, and it is well separated 
geographically: I therefore give it specific rank. 
2. PAPILIO ARISTOLOCHIAE CEYLONICA. Also confined 
to Ceylon and found all over the Island. Variations 
approaching P. aristolochtiae are numerous, and specimens 
are occasionally found indistinguishable from that species. 
It is well separated geographically, but the differences are 
not permanent: I therefore treat it as a race, or sub-species. 
3. SURENDRA QUERCETORUM var. DISCALIS. The name 
discalis has been given to the Ceylon form. De Nicéville 
writes: ‘‘It is simply a small form of S. guercetorum ; it has 
absolutely, as far as I can see, no other character by which 
it can be distinguished.’’........ ““Even size is not 
sufficient to distinguish S. quercetorum, as I possess some 
specimens of that species from Sikkim, which are quite as 
small as any Surendra from S. India, Ceylon, or the 
Andamans. ”’ 
It varies appreciably in size in Ceylon. This form is 
neither permanent nor geographically, separated, and I 
therefore treat it as a variety. 
It is of course quite impossible to draw up any rule to fix 
at what stage in its development a variety becomes a race, 
or a race becomes a species. This must be left to the opinion 
