y to have acquired a knowledge of a
period of every 21 days. In the cases already given of the sheep which
travelled to their birth-place to cast their lambs, and the sheep in
Spain which know their time of march{291}, we may conjecture that the
tendency to move is associated, we may then call it instinctively, with
some corporeal sensations. With respect to direction we can easily
conceive how a tendency to travel in a certain course may possibly have
been acquired, although we must remain ignorant how birds are able to
preserve any direction whatever in a dark night over the wide ocean. I
may observe that the power of some savage races of mankind to find their
way, although perhaps wholly different from the faculty of birds, is
nearly as unintelligible to us. Bellinghausen, a skilful navigator,
describes with the utmost wonder the manner in which some Esquimaux
guided him to a certain point, by a course never straight, through newly
formed hummocks of ice, on a thick foggy day, when he with a compass
found it impossible, from having no landmarks, and from their course
being so extremely crooked, to preserve any sort of uniform direction:
so it is with Australian savages in thick forests. In North and South
America many birds slowly travel northward and southward, urged on by
the food they find, as the seasons change; let them continue to do this,
till, as in the case of the sheep in Spain, it has become an urgent
instinctive desire, and they will gradually accelerate their journey.
They would cross narrow rivers, and if these were converted by
subsidence into narrow estuaries, and gradually during centuries to arms
of the sea, still we may suppose their restless desire of travelling
onwards would impel them to cross such an arm, even if it had become of
great width beyond their span of vision. How they are able to preserve a
course in any direction, I have said, is a faculty unknown to us. To
give another illustration of the means by which I conceive it _possible_
that the direction of migrations have been determined. Elk and reindeer
in N. America annually cross, as if they could marvellously smell or see
at the distance of a hundred miles, a wide tract of absolute desert, to
arrive at certain islands where there is a scanty supply of food; the
changes of temperature, which geology proclaims, render it probable that
this desert tract formerly supported some vegetation, and thus these
quadrupeds might have been annually