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of Kenya and New Zealand, the animals suffer from a progressive
anaemia; they grow thinner and thinner, and finally lose control of
their limbs. In certain parts of the plains region of the United States
and Canada, on the other hand, iodine is the defaulter, and farm
animals (like the human population) suffer from the swelling of the
thyroid known as goitre, with all the attendant symptoms of low
chemical activity and stunted growth. In some areas, the lack of
iodine is so pronounced that the young pigs lose all their hair and
hardly any of them survive.
The shortages, as we have said, are primarily due to a deficiency
native to the soil. It is surprising but true that there are great
stretches of country which from the outset are unsuitable (without
special treatment) for stock-raising on any large scale, because the
ground simply does not have enough of one or another chemical
element. Countries composed of igneous rock often have a shortage
of calcium. In much of the west of Scotland, where the soil is poor in
calcium and phosphorus and the pastures have long been depleted
by grazing without any return in the shape of artificial manure, the
sheep are frequently afflicted with disease, there is a high rate of
mortality among growing lambs, and the carrying capacity of the
land is falling. Iodine is generally low in limestone districts, or
where, as in parts of North America, the great meltings that followed
the Ice Age have leached it out of the soil
Phosphorus is the trickiest of all these elements. It is the one which
usually is nearest to the border line, and there are very big tracts of
phosphorus-poor soil. In addition, drought apparently makes it
harder for plants to get phosphorus out of the ground, so that an arid
climate will turn a soil that elsewhere would be adequate into a
phosphorus-deficient one.
Why, then, are these regions of the earth's surface not bare of wild
animals? And how is it that man can generally thrive where his
cattle sicken? The answer is that the demands are a matter of degree.
No region is entirely without any of the essential elements. In nature,
a balance is soon struck. The country supports what it can support.
If animals fall sick, they are speedily eliminated; as soon as over-
multiplication of any grazing animal brings down the supply of any
element per individual to the danger point, migration relieves the
pressure. Man, on the other hand, attempts more intensive opera-
tions. He wants the land to carry the maximum amount of stock,
and to carry it all the time. Furthermore, different animals make