52 
The habit, which, in Britain, confines the polyphagous Clisiocampa 
castrensis to our coast marshes whilst the almost equally polyphagous and 
closely allied C. neustria is generally distributed in gardens and hedge¬ 
rows, results in their almost complete isolation. The habit that confines 
Nudaria senex to marshy meadows, and N.mundana to the neighbourhood 
of old walls; the habit that confines Spilosoma urticae to marshes, and 
allows S. menthastri to abound in gardens and hedgerows everywhere; 
the habit that makes Dasychira fascelina a coast, and 9. pudibunda a 
woodland, species, are sufficient to illustrate another means by which 
isolation between species occupying practically the same tract of 
country may be brought about. 
DIFFERENCE OF HYBERNATING HABIT TENDING TO ISOLATION. 
Other habits that result in the isolation of closely allied species 
occurring in the same districts, and having the same food-plants, are 
well illustrated in our fauna. Toxocampa pastinum hybernates as a 
larva, its close ally, T. craccae, hybernates as an egg, yet both feed on 
Vida. Boaiinia consortana hybernates as a pupa, B. roboraria 
hybernates as a larva, yet both species are still restricted to oak. It is 
quite evident that, even should hybridisation take place between such 
species as these, the progeny must die, so different are the con¬ 
ditions, under which the vital processes, connected with the early stages of 
the two species, are respectively carried on, and every entomologist knows 
how fixed is the hybernating habit in nature for almost every species. 
DOUBTFUL ISOLATION. 
Where closely allied species, such as Leucania impura and L. 
straminea, occur at the same time, and on the same ground, and only 
partially, perhaps, specialised as to food-plant, it is possible that, in their 
early stages of separation, they were much more specialised as to 
habitat than they are now, for it is usually the case, as is evident in 
the example here given, that one of the two species (L. straminea) is 
strictly localised as to habitat, whilst the other (L. impura) is of more 
general distribution; for whilst the former is strictly confined to 
marshes, the latter occurs in meadows, woodlands, in fact, almost 
everywhere. It is possible, too, that even these two species are 
strictly specialised as to food-plant, L. straminea to reed ( Phragmites ), 
and L. impura to Carex. I am not sure whether these two species 
cannot easily discriminate each other by what may be a true recognition 
mark, for, in the meadows bordering reed beds, where both sometimes 
occur in profusion, I can separate them at night as they are flying, 
with the greatest ease, the white colour of L. straminea making that 
species very conspicuous. 
ISOLATION BY DIVERSE HABITS. 
In some cases, then, allied species may have been differentiated 
from a common stock by the development of diverse habits, as well as 
by a difference of form or colour, and the development of different 
habit—necessitating, as it may do, specialisation with regard to a par¬ 
ticular food-plant, hybernating in a different stage, appearing at a 
different time of the year, becoming single- instead of double-brooded, 
or vice versa, etc.—is almost sure to be correlated with a difference of 
colour, form or structure, for it undergoes its metamorphoses under 
different conditions, and we know now that a difference in the nutri¬ 
tive valueof the food given, and the formation of the wing pigments under 
varying conditions, etc., arc accompanied by distinct changes of size 
