Jim asks:

>Could you expand on this, and perhaps provide references?  Also, what
>do you mean by "economic selection" as the causal agent in all this?
>What did "fit" mean, pre-Fisher?  For that matter, what did it mean,
>pre-Spencer?

"Fitness" meant "appropriateness to purpose" to Darwin.
Outside of Fisherian
mathematical biology, it means the same thing today, and is written in one
fashion or another on every software warranty. This wording accompanies a Linux
distribution::

"This software is provided by the Regents and contributors "as
is" and any
express or implied warranties, including, but not limited to, the implied
warranties of merchantability and *fitness for a particular purpose* are
disclaimed."

Darwin used the word in exactly the same way. You can get a deep sense of
Darwin's definition just from his writings. These sentences appear in Chapter 6
of the "Origin":

"Furthermore, we may conclude that transitional grades between structures
*fitted for very different habits of life* will rarely have been developed at
an early period in great numbers and under many subordinate forms."

"Although no graduated links of structure, *fitted for gliding* through the
air, now connect the Galeopithecus with the other Lemuridae, yet I can see no
difficulty in supposing that such links formerly existed..."

"When either case occurs, it would be *easy for natural selection to fit the
animal*, by some modification of its structure, for its changed habits, or
exclusively for one of its several different habits."

"But by far the most important consideration is that the chief part of the
organisation of every being is simply due to inheritance; and consequently,
though each being assuredly is *well fitted for its place in nature*, many
structures now have no direct relation to the habits of life of each species."

"So we may believe that the progenitor of the seal had not a flipper, but *a
foot with five toes fitted for walking or grasping*..."

Wirt Atmar
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