In Variation under Domestication, Darwin makes several references to the concept of "correlation of variation":
I will here only allude to what may be called correlated variation. Important changes in the embryo or larva will probably entail changes in the mature animal.
Some instances of correlation are quite whimsical; thus cats with blue eyes are invariably deaf.
In the chapter's ending, he also calls this "correlation of growth":
Variability is governed by many unknown laws, of which correlated growth is probably the most important.
I gather that this is Darwin's humble way of admitting that there are many unknown unknowns, at least as far as correlation is concerned.
But, since 1859 until present day, what did we find out about this quirky aspect of evolution? Are all cats with blue eyes deaf?