1893.] Evolution in the Genus Megascops. 643 
Part II. 
CAUSES AND INFLUENCES. 
The close relationship existing between various branches of 
science, is, perhaps, nowhere more clearly shown than in the 
present field of research. Four distinct causes have been 
found appearing to influence the condition of the screech owls, 
each of which has an evident bearing upon the other, and the 
whole forms such a chain of evidence that its truth can hardly 
fail to be apparent. 
These causes or influences are— 
1. Humidity. 
2. Temperature. 
9. Acquired characters. 
4. Forest areas. 
These will be treated separately under their respective head- 
ings and in the order given, while a careful examination of 
the maps will show their bearing upon the distribution of the 
color phases of the screech-owl. 
A.—INFLUENCE or HUMIDITY. 
It has been conclusively shown by Allen? that humidity is 
one of the main influences governing the local variations of 
color in individuals of the same species, and that the distribu- 
tion of the light colored races is strictly coincident with the 
regions of mean minimum rainfall, while the dark forms are 
confined mainly to regions of mean maximum rainfall. 
Naturally enough, its effect is greatest during the breeding 
season, and with the present species the months of March, 
April, May and June may be considered as the period during 
which the young would be most under its influence; accord- 
ingly, map 3 has been constructed upon the mean of the data 
for these four months. 
The shaded area designates that region where the mean 
humidity is 70 and over, while localities having less are 
unshaded. Now it will be seen that to a very great extent the 
grays are confined to those areas where the humidity is the 
3 Bull. Mus. Comp. Zool. II, 1871, 240-241. 
