pt them to slightly
different conditions of life; which can be differentiated from other
allied assemblages; which reproduce their like, and which usually breed
together--we require a fresh set of experiments calculated to determine
the matter of fact,--whether such species crossed with their near allies
do always produce offspring which are more or less sterile _inter se_.
Ample materials for such experiments exist, in the numerous
"representative species" inhabiting distinct areas on a continent or
different islands of a group; or even in those found in the same area
but frequenting somewhat different stations.

To carry out these experiments with any satisfactory result, it will be
necessary to avoid the evil effects of confinement and of too close
interbreeding. If birds are experimented with, they should be allowed as
much liberty as possible, a plot of ground with trees and bushes being
enclosed with wire netting overhead so as to form a large open aviary.
The species experimented with should be obtained in considerable
numbers, and by two separate persons, each making the opposite
reciprocal cross, as explained at p. 155. In the second generation these
two stocks might be themselves crossed to prevent the evil effects of
too close interbreeding. By such experiments, carefully carried out with
different groups of animals and plants, we should obtain a body of facts
of a character now sadly wanting, and without which it is hopeless to
expect to arrive at a complete solution of this difficult problem. There
are, however, some other aspects of the question that need to be
considered, and some theoretical views which require to be carefully
examined, having done which we shall be in a condition to state the
general conclusions to which the facts and reasonings at our command
seem to point.

_Sterility due to changed Conditions and usually correlated with other
Characters, especially with Colour._

The evidence already adduced as to the extreme susceptibility of the
reproductive system, and the curious irregularity with which infertility
or sterility appears in the crosses between some varieties or species
while quite absent in those between others, seem to indicate that
sterility is a characteristic which has a constant tendency to appear,
either by itself or in correlation with other characters. It is known to
be especially liable to occur under changed conditions of life; and, as
such change is usually the star