ife behaviour of certain mammals, and of
its existence much lower in the scale of life evidence is not wanting;
from which we can infer that it is not of recent origin, but that the
conditions in the external environment demanded such a system at a
remote period of avian development. Now even in its incipient stages the
system must have involved a separation of the sexes, and howsoever
slight the degree of separation may have been in comparison with that
which can be observed to-day, inasmuch as the power of locomotion was
then less highly developed, mating could only have proceeded
satisfactorily providing that males fit to breed had some adequate means
of disclosing their positions. Thus there is reason to think that from
the very commencement of the process variations of emotional disposition
expressed through the voice would have been of survival value.

But expressed in what direction, in loudness and persistency of
utterance, these are the qualities which, I imagine, would have been
more likely to have facilitated the search of the female? Yet if she
were uncertain as to the owner of the voice, neither loudness nor
persistent repetition would avail much; and as species multiplied and
the competition for the means of living became increasingly severe, so
the necessity of a territory would have become intensified, and so, too,
with the extension of range, would the separation of the sexes have been
an ever-widening one; and as with their multiplication, irregularities
and delays in mating, arising from the similarity of the calls, would
have increased in frequency, so a distinctive call, which would have
tended to minimise these risks, would have come to possess biological
value.

Here we have a theory of origin, but origin of what? Of certain
characteristics of song--nothing more; and therefore to suppose that it
furnishes a complete explanation, which satisfies all the requirements
of scientific logic, of so wonderful an intonation as that, for example,
of the Marsh-Warbler, or that no other relationships, except that of the
territory, enter into the total emotional complex, simplifying here or
elaborating there to meet the exigencies of diverse circumstances--to
suppose this would be foolish. That there are many relationships which
even to-day are leading to modifications in important particulars, but
which at the present time are beyond our cognisance, of this there can
be no doubt.

There is one process by w