duces offspring after its own kind. Under no
conditions would we expect a duck to lay an egg from which could hatch
anything but a duck. No Plymouth Rock chicken mated with another of
her own kind will ever lay an egg that will produce a Rhode Island
Red. We may believe that the dog has descended from some form of wolf,
but it is not meant that at any particular time in the past any wolf
mated with a wolf ever produced pups that were anything but wolves.

Why this should be so is one of the most profound problems of biology.
Nothing but the fact that the process has gone on under our eyes for
so long a time could blind us to its marvelous character. To open the
egg of a chicken and examine it by the most refined methods known to
science is to find in it absolutely nothing that could be by the
widest stretch of the imagination considered anything like a chicken.
The biologist who has examined such eggs before and knows them in all
stages of the process may recognize in an egg which had been incubated
for a short time something which his previous experience tells him
will become a chicken. But it has not the faintest resemblance to a
chicken until later in its development. In early spring one may gather
pond snails from any country stream and place them in an aquarium. The
change from the cold water on the outside to the warmer water of the
aquarium and the temperate climate of the room hastens the process
which in the stream would not take place until later. In a short time
one may find fastened to the glass side of the aquarium the little
mass of transparent jelly which surrounds and protects the delicate
eggs of these creatures. Fastened as they are it is easy to direct a
magnifying glass so as to observe the change which goes on within
these transparent eggs. It is even possible to apply a microscope in
such a way as to watch the transformation under the low power of the
glass. At first the eggs are as clear as water, having at the center a
slightly yellowish spot. This central mass divides and subdivides
until the separated sections grow so small and numerous as to lose
individuality. Then the mass begins to press out here and dent in
there. After a little while a double line of fine, hairlike
projections runs around the creature. These hairs wave in such fashion
as to make the embryo snail revolve slowly in its egg. A little later
and swellings become more pronounced over the surface. One side
flattens; the rotary m