casion to mention, for taking its
place among both the productions of our Alpine heights and of our
sea-shores, it will be found that in proportion as its habitat proves
ungenial, and its stems and leaves become dwarfish and thin, its little
white cruciform flowers increase, till, in localities where it barely
exists, as if on the edge of extinction, we find the entire plant
forming a dense bundle of seed-vessels, each charged to the full with
seed. And in the gay meadows of Orkney, crowded with a vegetation that
approaches its northern limit of production, we detect what seems to be
the same principle, chronically operative; and hence, it would seem,
their extraordinary gaiety. Their richly-blossoming plants are the poor
productive _Irish_ of the vegetable world;[24] for Doubleday seems to
be quite in the right in holding that the law extends to not only the
inferior animals, but to our own species also. The lean, ill-fed sow and
rabbit rear, it has been long known, a greatly more numerous progeny
than the same animals when well cared for and fat; and every horse and
cattle breeder knows, that to over-feed his animals proves a sure mode
of rendering them sterile. The sheep, if tolerably well pastured,
brings forth only a single lamb at a birth; but if half-starved and
lean, the chances are that it may bring forth two or three. And so it is
also with the greatly higher human race. Place them in circumstances of
degradation and hardship so extreme as almost to threaten their
existence as individuals, and they increase, as if in behalf of the
species, with a rapidity without precedent in circumstances of greater
comfort. The aristocratic families of a country are continually running
out; and it requires frequent creations to keep up the House of Lords;
while our poor people seem increasing in some districts in almost the
mathematical ratio. The county of Sutherland is already more populous
than it was previous to the great clearings. In Skye, though fully
two-thirds of the population emigrated early in the latter half of the
last century, a single generation had scarce passed ere the gap was
completely filled; and miserable Ireland, had the human family no other
breeding-place or nursery, would of itself be sufficient in a very few
ages to people the world.

We returned, taking in our way the cliffs of Marwick Head, in which I
detected a few scattered plates and scales, and which, like nine-tenths
of the rocks of Orkney,