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instead of keeping pace with the weight; and the same is true of the
heart. It would take us too far to go into the detailed reasons for
this; but the fact that a large animal does not need a brain or heart
of the same porportional size as a small model of the same type is
important. It warns us not to be too hasty in drawing conclusions as
to intelligence from percentage brain-weight, or as to the efficiency of
circulation from percentage heart-weight. Size itself reduces the per-
centage weight; we must know the proper formula before we can tell
whether an individual, a sex, or a species has a brain-weight effectively
above or below that of another individual sex, or species of different
magnitude. In man, comparisons (often invidious) have frequently
been made between the brain-size of men and women; but not until
Dubois and Lapicque worked out the proper formulae for change of
brain-proportion with size was it possible to say whether the smaller
brain of women meant anything save that the bodies of women were
Another such example, but of a rather different type. We marvel
at the size of an ostrich's egg, which would provide a large party with
breakfast, and is the equivalent by weight of about twenty hen's eggs.
But we forget to marvel at the ostrich itself, which weighs as much as
about forty or fifty hens. The size of birds5 eggs, in fact, does not
increase as fast as the size of the birds that lay them. A humming-
bird lays an egg 15 per cent, of its own weight; that of a thrush is
9 per cent., that of a goose some 4 per cent., and that of an ostrich
only 1*6 per cent. Two competing forces are here at work. It is
advantageous to have large eggs, since they give the young bird a
better start in life; but the purely physical fact that all the new
material for the egg's enlargement must pass through the egg's surface
-will, as bulk grows, slow down egg-increase below body-increase.
And, as a matter of fact, we find that in quite small birds, below the
size of a goose or swan, egg-weight increases only a little faster than
These figures apply to averages only. Adjustments can be made
in response to special needs. In wading birds the young must run
about immediately on being hatched; and accordingly their egg-size
is well above that of equal-sized birds whose young are born naked
and fed in the nest. The common cuckoo, to deceive its hosts, must
have an egg not too unlike theirs in size; and accordingly its egg is
uniquely small—appropriate to a bird one-third of its body-weight.
The limitation of egg-size is prescribed by laws which apply to dead
as well as to living matter; its regulation within these inexorable
limits is the affair of the interplay of biological forces.