And all these additions to, and readjustments of, the nerve-centres
must take place without any disturbance of the other necessary
adjustments already attained. This is no simple problem.
We will here neglect the fact that many other changes are going on
simultaneously. Legs are being formed or moulded into jaws, the
anterior segments are fusing into a head, and their ganglia into a
brain; an external skeleton is developing. Furthermore the increase
of the muscular and nervous systems must be accompanied by increased
powers of digestion, respiration, and excretion. Practically the
whole body is being recast. We insist only on the necessity of
simultaneous and parallel changes in muscles, nerves, and
nerve-centres; though what is true of these is true, in greater or
less degree, of all the other organs.
You may answer that this is to be explained by the law of
correlation of organs; that when changes in one organ demand
corresponding changes in another, these two change similarly and
more or less at the same time and rate. But this is evidently not an
explanation but a restatement of the fact. The question remains,
What makes the organs vary simultaneously so as to always correspond
to each other? The whole series of changes must to some extent be
effected at once and in the same individual, if it is to be
preserved by natural selection.
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