y do not use
lures or disguises, or traps or poison.

There is, of course, no limit to the variety and adaptiveness of
nature taken as a whole, but each species is hedged about by
impassable limitations. The ouzel is akin to the thrushes, and yet it
lives along and in the water. Does it ever take to the fields and
woods, and live on fruit and land-insects, and nest in trees like
other thrushes? So with all birds and beasts. They vary constantly,
but not in one lifetime, and the sum of these variations, accumulated
through natural selection, as Darwin has shown, gives rise, in the
course of long periods of time, to new species.

As I have already said, domestic animals vary more than wild ones.
Every farmer and poultry-grower knows that some hens are better with
chickens than others--more motherly, more careful--and rear a greater
number of their brood. The same is true of sows with pigs. Some sows
will eat their pigs, and wild animals in cages often destroy their
young. Some ewes will not own their lambs, and occasionally a cow will
not own her calf. (Such cases show perverted or demoralized
instinct.) Similar to these are the strange friendships that sometimes
occur among the domestic animals, as that of a sheep with a cow, a
goose with a horse, or a hen adopting kittens. In a state of nature
these curious attachments probably never spring up. Instinct is likely
to be more or less demoralized when animal life touches human life.

With the wild creatures we sometimes see one instinct overcoming
another, as when fear drives a bird to desert its nest, or when the
instinct of migration leads a pair of swallows to desert their
unfledged young.

A great many young birds come to grief by leaving the nest before they
can fly. In such cases, I suppose, they disobey the parental
instructions! I find it easier to believe that instinct is at fault,
or that one instinct has overcome another; something has disturbed or
alarmed the young birds, and the fear of danger has led them to
attempt flight before their wings were strong enough. Once, when I was
climbing up to the nest of a broad-winged hawk, the young took fright
and launched out in the air, coming to the ground only a few rods
away.

Instinct, natural prompting, is the main matter, after all. It makes
up at least nine tenths of the lives of all our wild neighbors. How
much has fear had to do in shaping their lives and in perpetuating
them! And "fear of any particular