= E 
for which disintegration is necessary 1f the path is to be cleared 
Those who have resorted to the process of experiment must often 
have been astonished at the completeness with which a very sim- 
ple one has sometimes disposed of a theory having much to 
recommend it. 
Seasonal Dimorphism. 
SUMMER AND WINTER PHASES. ASPECT AND LIFE-HABIT. 
I think I can best justify the occupation by me of the time and 
thoughts of members of this International Congress, if, instead of 
any further remarks on the general subject of experimental ento- 
mology, I proceed to the application which I have been able to 
make of experiment for the purpose of throwing light on the fascin- 
ating subject of the factors in seasonal dimorphism. For many 
vears, though with much interruption owing to the pressure of 
other demands, I have been engaged in experiment, with a view 
to ascertain some of these factors and their relative importance. 
The facts they present are sufficiently remarkable. Between two 
successive generations the differences are so great that, but for 
knowledge of their parentage, the succeeding generation would 
often be taken for a different species altogether. I have found it 
expedient for the purpose of this exposition to divide these differ- 
ences into two classes. One class of differences, all which I 
include in the general term « aspect », comprises general colour- 
ing, pattern or markings, shape, and size (this latter I would 
rather consider as « mass », since bulk is in a sense more significant 
than expanse, and is also much more easily ascertained by weigh- 
ing); the other, and as I shall venture to submit the more fun- 
damental class of differences, may be summed up in the term 
« life-habit ». The species on which most of my experiments have 
been tried are the well-known Araschnia levana and the common 
Selenia bilunaria, the one generation of each species appearing as 
an imago in the spring or very early summer, the other in the 
height of summer. 
Let me now mention the names by which I propose in this 
paper to distinguish these two generations. They are commonly 
spoken of as the first and second broods. I am going to adopt for 
them the terms winter phase and summer phase. The form com- 
monly called the first « brood » is really the second brood of the 
