360 THE STRUCTURE OF EVOLUTIONARY THEORY
Morphological "degeneration" provides the best example of environmental
impetus within Eimer's concept of balance between internal and external forces.
The ontogenetic channel supplies the internal component, since simplification can
be achieved most easily by backing down the ontogenetic pathway. But
environment must provide the downward push—for change usually proceeds in the
other (and upward) direction, as the biogenetic law asserts. Environment, in this
case, might act in a positive sense (when simplification leads to local adaptation, as
in parasitism), or in a more negative way (when unfavorable climates prevent the
full passage of ontogeny, and a resulting juvenilization then becomes inherited in
Lamarckian fashion). Eimer writes: "The abundance of the species which have
been formed by degeneration, by retrogression, is known to every zoologist. It is
self-evident that their origin is to be traced to the action of external conditions"
(1890, p. 53).
Eimer found his best example of balance, or environmentally triggered
orthogenesis, in a favorite case of all early evolutionists—the Mexican axolotl, or
sexually mature tadpole of the genus Ambystoma. (Axolotl formerly bore the
separate generic name Siredon, while the parental form often received a misplaced
"1" as Amblystoma—as in the quote below.) Ontogeny set the orthogenetic channel
and change could only proceed in the prefigured direction, up or down. But the
actual path taken depends upon the environment—down for warm and permanent
ponds, up in more terrestrial climates:
Siredon is for the problem of evolution one of the most important living
animals, in that it brings so beautifully before our eyes the transition of
lower sexually mature into a higher sexually mature form, and at the same
time shows so clearly the causes of the transformation. We discern these
causes simply in the reaction of the organism under external conditions, the
increased exertion of an organ already in process of formation (the lung),
and the disappearance of another (gills) in consequence of definite demands
of the environment, and changes connected with these by correlation.
Amblystoma appears where the axolotl has too little water to live in, where
it is compelled to live on dry land. That this is the case is proved by the
possibility of artificially rearing Amblystoma from the axolotl by gradually
withdrawing water (1890, p. 46).At this point, Darwinian forces might have played a major role in Eimer's
system. He needed external potentiators to trigger prefigured orthogenetic
pathways. Had he chosen selection as a preferred external force, his resulting
amalgam would still have departed from a Darwinian rubric—for the idea of
internal orthogenetic channels runs so contrary to the key Darwinian postulate of
undirected variation. But selection would still have played an important role as an
instigator of trends.
However, Eimer chose Lamarckism as the preferred external potentiator for
his "brand" of functionalism. He opted for use and disuse with inheritance of
acquired characters as the standard mode for transferring environmental
information to organisms—and thus as the primary mechanism for