1928] 
The Bee that Works in Stone 
79 
state that P. crawfordi Ckll. has never been taken away from the 
environs of the salt flats west of Lincoln, Nebraska. Perhaps in 
this as well as in other cases, similar geological limits are set on 
the bee, the same as on P. opuntice. Further investigation of 
this point would probably be interesting. 
Various investigators have found evidences of adaptation 
to new conditions, of extraordinary splitting up into a great 
variety of forms and, in general, of considerable signs of evolution 
going on within this genus. Professor Cockerell thus states “we 
have indeed the process of evolution going on under our eyes, 
the puzzling forms being those which have only lately segregated 
themselves and have not yet developed striking peculiarities.’ ’ 
Grsenicher brings forth some evidence when he states that P. 
maculipennis obtains her pollen from the white melilot which 
was introduced from Europe. Here there occurred an adapta- 
tion to new conditions brought about by the fact that the 
native plant or plants visited originally by this bee did not occur 
in the type locality at the time the bee was found. 
With these facts in mind, Professor Cockerell has advanced 
the following five possibilities of processes that may be going 
on in the formation of new species in the genus Perdita : 
1. Mutations having no adaptive significance. 
2. Results of the crossing of mutants. 
3. The sorting out of certain characters as dominant (in 
the sense of prevalent) but not necessarily aided by 
natural selection or sexual selection. 
4. The occasional coincidence of adaptive features (often 
physiological or such morphological ones as length of 
tongue) which favor a change of habits or environment 
and permit the insect to become attached to a different 
genus of plants. 
5. The new type having been isolated on a new plant, or 
geographically or seasonally isolated will eventually 
settle down to a new position of stability (aided by 
natural selection) which will be sufficiently remote 
from that of the parent species to maintain it as a 
distinct entity in nature, and usually prevent crossing* 
