Baldwin suggested sea level as a kind of metaphor for
this: Above sea level is what selection can get to, and
what selection can't get to , think of it as erosion
in the body, and this is the expression of the genotype.
You can unmask selection by putting animals
in difficult spots and then lots of things that normally
wouldn't have been selected suddenly become
available for natural selection to pick at and work at.
Baldwin suggested that, in a sense, sea level rises and
a whole lot of stuff can now freely vary, so you can
sample a wider space because of your learning ability,
or because of your plasticity.
Waddington, on the other hand, put animals in a
very much more stressful environment, an environment
full of ether, for example, or an environment that
was very hot, and showed that you can also unmask a
lot of variants and subject those to selection.