mental power is its performance commonly found to be in respect of
its own limited and special instinctive department. This holds as
good with the lower animals as with men, and is explained by the fact
that perfection of proficiency is only partly dependent upon natural
capacity, but is in great measure due to practice and cultivation of
the original faculty. A philologist, for example, is unskilled in
questions of jurisprudence; a natural philosopher or mathematician,
in philology; an abstract philosopher, in poetical criticism. Nor
has this anything to do with the natural talents of the several
persons, but follows as a consequence of their special training. The
more special, therefore, is the direction in which the mental
activity of any living being is exercised, the more will the whole
developing and practising power of the mind be brought to bear upon
this one branch, so that it is not surprising if the special power
comes ultimately to bear an increased proportion to the total power
of the individual, through the contraction of the range within which
it is exercised.

Those, however, who apply this to the elucidation of instinct should
not forget the words, "in proportion to the entire mental power of
the animal in question," and should bear in mind that the entire
mental power becomes less and less continually as we descend the
scale of animal life, whereas proficiency in the performance of an
instinctive action seems to be much of a muchness in all grades of
the animal world. As, therefore, those performances which
indisputably proceed from conscious deliberation decrease
proportionately with decrease of mental power, while nothing of the
kind is observable in the case of instinct--it follows that instinct
must involve some other principle than that of conscious
intelligence. We see, moreover, that actions which have their source