ESA 
to either phase, in all respects; that is to say, both as regards 
aspect, including size and therefore weight or mass, and life-habit, 
and even to divert the Insect, after being launched in the direction 
of one phase, into the other. 
To avoid constant reference to the thermometer with its two 
scales (Fahrenheit and Centigrade), I have classified the different 
temperatures used in mv experiments as follows : 
1. « Spring » temperature, a mean between 50° F. (10° C.) and 
50°-F (13° C.), usually about 54° E (12° C.) te 56° E. (13%C.); 
2. « Cool summer » temperature 60°-62° F. usually about 60° F. 
(15°-16° C.); 
3. «Full'summer » temperature, 65° Huesos Ci); 
4. « Forcing » temperature, 75°-80° F. (24°-27° C.). 
My temperatures are mean temperatures, obtained by shifting 
the objects from time to time, between a refrigerator usually 
about 46° F. (8° C.) and a room at a temperature of about 
66° F. (19° C.), varying the time of exposure — usually once 
in 24 hours, but sometimes once in 2 or 3 days or even more — to 
suit the mean temperature desired. This is all that my opportun- 
ities permitted; the temperatures so arrived at can only be 
considered approximate; and it does not follow that exposure for 
6 hours at 40° F. and 18 hours at 60° would have the same operative 
effect as 12 hours at 50° and 12 at 60°, though the mean would be 
the same. But it is more nearly in accordance with nature than a 
fixed temperature would have been, because, as we know, there is 
almost always a great difference between the day and night, 
often amounting to 20 degrees F.(11 degrees C.) or more, especially 
in the summer months. 
NOT « ALTERNATION OF GENERATIONS ». 
It has sometimes been suggested that the cause of difference 
between the two phases is a congenital one, that under ordinary 
circumstances, where seasonal dimorphism occurs, it is a case of 
alternation of generations. This is certainly incorrect as regards 
the two species on which most of my experiments have been tried, 
A. levana and S. bilunaria. Of the latter I have had four succes- 
sive generations within about six months, all of the pure summer 
