categories, such as mammals and birds, with few or no intermediate forms,
but the actual individuals most closely similar to one another naturally
and inevitably fall into distinct groups which we call kinds or species.
The conception of a species is difficult to define, and authorities are
not agreed about it. Some, like Professor Huxley, state that a species is
purely a mental conception, a generalised idea of a type to which actual
individuals more or less closely conform. According to Huxley, you cannot
lock the species 'horse' in a stable. Others regard the matter more
objectively, and regard the species merely as the total number of
individuals which possess a certain degree of resemblance, including, as
mentioned above, all the forms which may be produced by the same parents,
or which are merely stages in the life of the individual. There are cases
in which the limits of species or the boundaries between them are
indistinct, where there is a graduated series of differences through a
wide range of structure, but these cases are the exception; usually there
are a vast majority of individuals which belong distinctly to one species
or another, while intermediate forms are rare or absent. The problem then
is, How did these distinct species arise? How are we to explain their
relations to one another in groups of species or genera; why are the
genera grouped into families, families into orders, orders into classes,
and so on?

There are thus two main problems of evolution: first, how have animals
become adapted to their conditions of life, how have their organs become
adapted to the functions and actions they have to perform, or, at least,
which they do perform? The power of flight, for example, has been evolved
by somewhat different modifications in several different types of animals
not closely related to one another: in reptiles, in birds, and in mammals.
We have no reason to believe that this faculty was ever universal, or that
it existed in the original ancestors. How then was it evolved? The second