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There are many other ways in which the big animal inevitably
fails to be a mere scale enlargement of its smaller relatives. The
relative size of many organs decreases instead of increasing with total
absolute bulk, so that in a big animal they do not have to be pro-
portionately so large as in a small one. Relative wing-size is a case
in point.
Then everyone knows the small-eyed look of an elephant, or, still
more, of a whale. To obtain a good image, an eye has to be of a
certain absolute size: this is because the image even in our own eyes
is really a mosaic, each sensory cell in the retina behaving as a unit.
The image we see is built up out of unitary spots of colour, just as a
half-tone picture in a newspaper is built up out of combinations of
single black and white dots. To give an image of a reasonably large
field, they must be numerous. Once a certain absolute size of eye is
reached, any advantage due to further enlargement is more than
counterbalanced by the material used and the difficulties of construc-
tion, just as very little advantage is to be gained in photography by
making a camera over full-plate size. Even in a giraffe, which has an
exceptionally large eye for a big animal, the eye's relative weight is
small compared with that of a rat.
Most sense-organs behave in a similar way. This is especially true
of the organs of touch and temperature in the skin. It matters to a
mouse to be able to deal with things the size of breadcrumbs. But
such trivialities do not concern an elephant; the elephant accordingly
can, and does, have its skin sense-organs much more thinly spread
over its surface.
This in turn has an effect on the size of the nervous system; for the
fewer the sense-organs, the fewer sensory nerve-cells are needed, and
the smaller the size of the ganglia on the spinal nerve-roots which are
composed of sensory nerve-cells. Since the sense-organs of touch are
distributed over the surface, we should only expect these ganglia to
grow proportionately to surface, and not to bulk, even if the sense-
organs were as thickly scattered over the skin of a big as of a small
animal; but as they are more sparsely scattered in the big animal the
weight of a ganglion does not even keep up with the size of the ani-
mal's surface, and its growth is actually only just more than pro-
portional to the square root of the weight.
As a matter of fact, when the nervous system as a whole, or the
brain by itself, is compared in a series of related mammals or birds of
different size, it is found to increase only about as fast as the surface,