14 Dr. T. A. Chapman on 



easy, even inevitable, crossing must have been, had they 

 been merely forms of one species, I see no means of avoid- 

 ing the conclusion that the two forms are asyngamic. 

 Such an attitude towards each other, seemed to be regarded 

 as the truest test of specific distinctiveness of two forms, 

 when the subject of " what is a species " was debated at 

 our meeting last Spring. The difficulty is to put it to 

 experiment. In the case of Ercbia palarica nature has 

 provided the experiment, and the answer is distinct. 

 Usually she separates the subjects of experiments so 

 widely by time, place, and season, that the answer we have 

 to form is a purely personal one, viz. / think if the experi- 

 ment were made the answer would be so and so. It was 

 in this way that I concluded, and still hold that all the 

 Spanish forms of stygne previously known to me are of 

 one species. 



When we examine the insects themselves for confirma- 

 tion of this conclusion, we first have size, usually a poor 

 specific character, but here so marked and constant as to 

 have some weight. Then as to markings, the rusty blotches 

 in both species are divided into sections by the more or 

 less dark lines of the nervures. and each section in the 

 interneural spaces of the fore- wing has a definite form that 

 differs completely in the two species, and is constant in 

 every one of the large number I have examined. In stygne 

 the interneural blotch on its basal margin is stretched out 

 centrally into a more or less full convex margin, as if the 

 nervures carried the dark ground-colour into the blotch, 

 and so forced it to swell out into the intervals. In 

 palarica, each interneural blotch falls away from the base 

 in its mid-neural line, forming a more or less deep notch. 

 This is most marked in the blotch between nerves 3 and 4, 

 where the ocellus is weak or absent. This blotch is also 

 shorter than the others as if it failed with the ocellus, 

 whilst in stygne, though the ocellus may be wanting, this 

 blotch stretches inwards at least as strongly as the others. 

 The result is an hour-glass shape of the rusty mark on the 

 fore-wings, contrasting with the characteristic outline in 

 stygne. In the ^ the rusty blotch is continued up to the 

 costa, with a deflection base wards, by a whitish-grey shade, 

 of which any trace is wanting in stygne. The blotches on 

 the hind-wings have a similar character. The blotch of 

 each ocellus is much the same in both species when it is 

 reduced, but when it is pronounced and large, it differs 



