164 THE STORY-OF THE: BIRDS. 
been at times more extensive or less so in the bird’s 
past history. Even some dogs acquire them while 
others have not, and they are so easily influenced that 
some dry-land amphibians put them on at the social 
season when they go into the water and shed them 
when they leave. Many birds, as our Northern 
grouses, expand the same margin into a fringed 
snowshoe to use in winter to broaden their tread, 
and they shed it again in summer. A close study 
of webs is therefore interesting as bearing upon the 
history of the bird’s recent habits, but we can not fol- 
low it further here, except to say that since webs may 
come and go so easily, it is not at all probable that 
any bird inherited a webbed foot from any reptilian 
ancestor. So far as we can now see, it is more likely 
that most of the modifications of the feet for special 
uses were made within the birds by their own peculiar 
habits. 
It is true that the reptiles’ feet were also modified 
for similar purposes, showing how sensitive to use and 
environment the foot has always been. We have 
seen that such reptiles as walked upright (bipedally) 
had birdlike feet, rather like those of the ostrich forms 
of to-day; and it may be possible that tree-haunting 
habits had given some of them, that were immediate 
ancestors of the birds, opposable hind toes for limb 
grasping, just as the toes of the chameleons are 
bunched for this purpose, and as the first toe of some 
low mammals, as opossums and others, is opposable. 
The bird’s foot, however, shows every indication 
of being based upon the type of that of the lizards. 
