51 
to appear in the imaginal state at the same time, is as potent as any 
physical barrier, that prevents the two incipient species spreading to 
each other’s grounds. It is only isolation of the kind pointed out 
above that could allow of the formation of a new species, under the 
same general conditions, and on the same ground, as that occupied by 
its immediate progenitor. Given this isolation, the close inter¬ 
breeding of the individuals of the new form and the intensification 
of its peculiarities are as assured as is the powerlessness of the new 
to cross with the ancestral form. The formation of peculiar andro- 
conia and other characters will also tend to specialise the new form, 
and, when once the peculiarities of the new form, albeit at first 
modifications of the old form, have become fixed, the possibility of 
crossing frequently in nature, even if subsequent changes lead to their 
appearance at the same time, will be much lessened thereby. How 
strong a factor this may really be in the differentiation of closely 
allied species, will be evident to every field entomologist. Brenthis 
euphrosyne and Brenthis selene offer an illustration. In dozens of closely 
allied Noctuid moths its influence is evident, e.y., Ayrotis tritici and 
A. obeliscci, occurring in July-August, and late September respectively, 
Caradrina ambiyua and C. taraxaci, Ayrotis seyetum and A. lunigera, 
Triphaena subsequa and 2 7 . orbona (comes), etc. In none of these are 
there any real “recognition marks” in the true sense of the word. 
SPECIALISATION OF LEPIDOPTERA TO FOOD-PLANT. 
A difference of habit, which results in the isolation of allied 
species by causing the appearance of the imagines at separate times of 
the year, is, however, only one of a large number of habits which result 
in isolating closely allied species. There is, first of all, the specialisation 
of two allied species to two different food-plants of restricted range, 
or restricted to different geological formations. This not only tends 
to keep species very local, by confining them to the ground on which 
the food-plant grows, but it also isolates them from each other owing to 
the necessity of the insects, confined to a local plant for pabulum, giving 
up any wandering tendency they may have originally possessed. Species 
that have been or are isolated in this way, are numerous in our fauna. 
Thecla w-album is confined to elm ( Ulmus ), T. pruni to blackthorn 
(Primus), Cupido minima is confined to Anthyllis, Lycaena anon to 
thyme (Thymus), Nemeobius lucina to Primula, and Nisoniades tayes 
to Lotus; Sphinxpimastri is confined to Pinus, Macroglossa fuciformis to 
honeysuckle, and M. bombyliformis to scabious ; Adscita ylobulariae is 
confined to Centaurea, A. statices to Rumex, and A. geryon to Helian- 
themum, Sarrothripa undulana and Halias cblorana are confined to 
Salix, Hylophila bicolorana to oak, Leiocampa dictaea is confined to 
Salix and Populus, L. dictaeoides practically to Betula. Lophopteryx 
cuculla is confined to Acer, L. carmelita to Betida, Notodonta trepida 
is confined to Quercus, so also is Drymonia chaonia ; Cerura bifida 
affects Populus, C. furcula prefers Salix, and C. bicuspis feeds on Betida 
and Alnus. It is possible to show that about a third of the British 
lepidopterous fauna is more or less distinctly specialised, with regard 
to some particular food-plant, and is, therefore, more or less isolated 
from its allies equally specialised with regard to some other food-plant 
(or food-plants). 
ISOLATION BY DIFFERENCE OF HABITAT. 
Other habits may result in the isolation of closely allied species. 
