w which has regulated the
peopling of the earth with animal and vegetable life is, that
every change shall be gradual; that no new creature shall be
formed widely different from anything before existing; that in
this, as in everything else in nature, there shall be gradation
and harmony--then these rudimentary organs are necessary and are
an essential part of the system of nature. Ere the higher
vertebrates were formed, for instance, many steps were required,
and many organs had to undergo modifications from the rudimental
condition in which only they had as yet existed.... Many more of
these modifications should we behold, and more complete series of
them, had we a view of all the forms which have ceased to live.
The great gaps that exist ... would be softened down by
intermediate groups, and the whole organic world would be seen to
be an unbroken and harmonious system.

The article, in which we can see a great generalisation struggling to be
born, ends thus:

It has now been shown, though most briefly and imperfectly, how
the law that "every species has come into existence coincident
both in time and space with a pre-existing closely allied
species," connects together and renders intelligible a vast number
of independent and hitherto unexplained facts. The natural system
of arrangement of organic beings, their geographical distribution,
their geological sequence, the phenomena of representative and
substituted groups in all their modifications, and the most
singular peculiarities of anatomical structure, are all explained
and illustrated by it, in perfect accordance with the vast mass of
facts which the researches of modern naturalists have brought
together, and, it is believed, not materially opposed to any of
them. It also claims a superiority over previous hypotheses, on
the ground that it not merely explains but necessitates what
exists. Granted the law, and many of the most important facts in
nature could not have been otherwise, but are almost as necessary
deductions from it as are the elliptic orbits of the planets from
the law of gravitation.

Some time after the appearance of this article, Wallace was informed by
his friend and agent, Mr. Stevens, that several naturalists had
expressed regret that he was "theorising," when what "was wanted was to
collect more facts." Apart from t