30 BOTANICAL GAZETTE [JULY 
be a disadvantage before the optimum is reached, may be an 
advantage after the optimum is passed. Whatever may be the 
characteristic which enables this set of individuals to hold its 
own in a new ecological position, I think the principal circum- 
stance which accounts for the adoption of a new mode of life is 
the pressure of competition. The dominant form retains the 
original position, the other becomes modified (specialized) in 
adaptation to the newly acquired position. 
In my neighborhood there are thirty-five species of Andrena, 
which complete their flight from March 17 to July 14. These 
succeed one another, so that not more than twenty-one would 
be in competition at the same time, if their habits were the 
same. Ten begin their flight in March, seventeen in April, 
seven in May, and one in June. 
Of thirty-three species whose habits are pretty well knows, 
nineteen are polytropic and fourteen oligotropic, in the sens 
in which I use those terms. Four of the oligotropic species get 
pollen from plants of the same genus, but each of the other tel 
has its own flower, so there are eleven sets which are absolutely 
without competition among themselves. I think it is clear that 
so many species could hardly flourish in the same locality and 
complete their flight in so short a time, if all were in competition 
for the pollen of the same flowers. 
The average maximum flight of the females is forty-eight days 
Now suppose that, on account of the pressure of competition 
one of these shifts to a different phenological position. Of OF 
flowers whose pollen is so situated that the bee can readily col 
lect it, only those are available whose pollen is produced m 
abundance between the time the female is impregnated and the 
end of the time of flight. To use human terms, the bee mn 
choose between a limited number of flowers, and is in no wise iret 
to regulate its habits according to mere whim. oe 
From the above considerations I do not accept the views ® 
Kerner, although they are the ones adopted by Knuth? © 
* Natural History of Plants 2: 206. 1894. ae 
3Handbuch der Bliitenbiologie 1 : 106, 114. 1898. 
y ty 
