The Eggs of Birds 451 
adaptation follows more slowly. For example, let us 
take the group of owls. The majority of these birds nest 
in hollow trees, but even these occasionally make use of 
an open hollow or a very shallow one, and individual, 
radical departures from the conventional owl-habitation 
are doubtless not uncommon. But these exposed egys 
are soon destroyed; for no crow, Jay, or squirrel could ever 
resist any opportunity to avenge himself for the wrongs 
inflicted by his ancestral enemy, the owl. But when, 
urged on by that impulse which ever tends to make birds 
vary their habits in all directions, some owl, such as the 
Short-eared, finds good feeding on marshes and open, 
treeless plains, it naturally takes to nesting on the ground, 
in nests but partly concealed by the overhanging grasses. 
Three things might now happen. If sufficient varia- 
tion occurred and the conditions demanded it, natural 
selection might bring about a protective colour on the 
shells of the eggs; if enemies were few and easily over- 
awed, the eggs might remain white; while, on the other 
hand, the enterprising race might be wiped out of exist- 
ence for no more reason than the colour of the egg-shells. 
The second result seems to be the good fortune of the 
Short-eared Owls. All of these fates have undoubtedly 
overtaken birds again and again, and it is by the inter- 
action of such condition , combined with an ever-chang- 
ing environment, that many phenomena are brought about. 
It was by reason of the general similarity in colour 
which the eggs of related groups of birds tend to show 
to each other that odlogy, or the science of ege-shells, 
was able to initiate an important scientific discovery. 
