50 
that mutual infertility would be usually brought about by natural 
selection wherever the two forms were in contact; also that the early 
occurrence of well-marked differences would assist greatly in the 
rapidity of adaptation. 
VALUE OF RECOGNITION MARKS. 
The value of these “ recognition marks ” is probably of some 
importance in many species, yet the presence of such marks does 
not prevent the crossing of such species as Smerinthus ocellatus and 
S. populi, of Amphidasys strataria and A. betularia, of Ennomos 
quercinaria (angularia) and E. autumnaria, when opportunity offers. 
Still, there can be no doubt that, in Lepidoptera, specialisation of 
androconial scales, scent tufts, and similar structures may have much 
to do with the usual natural mating of species, especially in view of 
our present knowledge of the sight of insects, and the doubt that 
may naturally arise as to their want of ability to discriminate small 
and trifling colour marks, such as those which usually exist between 
two very closely allied species; nor is the colour sense of insects 
sufficiently defined to prevent the pairing of a perfectly normal specimen 
of A. betularia with a perfectly melanic aberration of the species, nor 
the correct pairing of the various forms of a polymorphic species like 
Cidaria immanata. It would appear certain, therefore, that the correct 
pairing of species in nature, among Lepidoptera, is often due to causes 
other than recognition marks, however important a part they may play 
in certain cases. 
ISOLATION. 
Although I do not see that mutual infertility would be, as Wallace 
asserts, brought about by natural selection, wherever two incipient 
species were in contact, yet it is quite clear that, in some way, nature 
must prevent their crossing, if a new form (or forms) is to be differ¬ 
entiated. Now, it is quite clear that, to prevent this, isolation of 
some kind must occur. After a careful consideration of the matter, it 
appears to me that, among Lepidoptera at least, the isolation is fre¬ 
quently, more or less perfectly, brought about by a difference in the 
time of year at which the imago reaches the perfect stage. Among our 
butterflies the single-brood of Limenitis sibylla falls between the two 
broods of the allied L. Camilla , and, in this case, there is not even a 
differentiation of the food-plants of the two species, both being con¬ 
fined to honeysuckle. Similarly, the single-brooded Polyommatus 
corydon falls between the two broods of P. bellaryus, nor must it be 
thought that these species are so distinct as the colour of their 
respective males would suggest, for Buckler and Hellins were both 
unable to definitely distinguish the larv®, whilst I have, in my 
possession, an undoubted wild hybrid of these two species. I need 
only call attention to the single-brooded Cidaria immanata, which 
occurs in July and August, at a time practically intermediate between 
the two broods of Cidaria russata ; to the parallel instance of the 
single-brood of Tephrosia crepuscularia, which appears between the two 
broods of Tephrosia bistortata. True, the emergences may occasionally 
overlap for a few days, locally, but for all intents and purposes, tho 
specios are as perfectly isolated as if their habitats wero separated 
by some of the most marked physical barriers. Isolation, indeed, 
must bo the essential factor of the differentiation of new species, and 
an isolation that is engendered by the physical inability of two species 
