Cuvier, and his British counterpart, Richard Owen, had an argument against evolution that you don’t hear very often anymore. Cuvier called it the laws of correlation, and it was the idea that organisms were fixed and integrated wholes in which every character had a predetermined value set by all the other characters present.
In a word, the form of the tooth involves that of the condyle; that of the shoulder-blade; that of the claws: just as the equation of a curve involves all its properties. And just as by taking each property separately, and making it the base of a separate equation, we should obtain both the ordinary equation and all other properties whatsoever which it possesses; so, in the same way, the claw, the scapula, the condyle, the femur, and all the other bones taken separately, will give the tooth, or one another; and by commencing with any one, he who had a rational conception of the laws of the organic economy, could reconstruct the whole animal.
Cuvier famously (and incorrectly) argued that he could derive the whole of the form of an animal from a single part, and that this unity of form meant that species were necessarily fixed. An organism was like a complex, multi-part equation that used only a single variable: you plugged a parameter like ‘ocelot’ into the Great Formula, and all the parts and pieces emerged without fail; plug in a different parameter, say ‘elephant’, and all the attributes of an elephant would be expressed. By looking at one element, such as the foot, you could determine whether you were looking at an elephant or an ocelot, and thereby derive the rest of the animal.
Darwin’s natural selection broke the narrow interpretation of the laws of correlation. He emphasized natural variation, the obvious observation that not all elephants or ocelots looked alike, and that individual parts can be plastic over time, and exhibit signs of adaptation. The creationism of people like Cuvier and Owen, though, was actually built on a rational, scientific proposition about the nature of organisms, and at least in the 19th century people could make principled, reasonable criticisms of evolution that built on prior science. Of course, one of the results of Darwin’s meticulous observations was that there is no one fixed shape for a species — you can hand a skilled anatomist a claw, for instance, and he might be able to derive ‘pigeon’ from it … but not ‘pouter’ or ‘fantail’ or ‘rock pigeon’. The results of the Great Formula are not quite so fixed as was thought.
Now some interpretations of Darwin go a little too far in presuming the organism is almost infinitely plastic, that selection can shape any one character in any direction and with a restricted scope — it can mold on part without complicating any other. We can, to a first approximation, pretend there is no Great Formula at all, no laws of correlation, just collections of characters aggregated in the organism that can be freely tweaked. This is going too far. Darwin himself, in The Origin, did not reject Cuvier’s Laws of Correlation at all, and actually had a wonderfully modern view of the relationship of the parts of an organism.
In looking at many small points of difference between species,
which, as far as our ignorance permits us to judge, seem quite
unimportant, we must not forget that climate, food, &c., have no doubt
produced some direct effect. It is also necessary to bear in mind
that, owing to the law of correlation, when one part varies, and the
variations are accumulated through natural selection, other
modifications, often of the most unexpected nature, will ensue.
There is a Great Formula for organisms, but it’s multivariate. There are many variables in the formula (I would go so far as to say that each gene is a separate variable, along with various environmental influences), the evaluation of the formula is a process called development, and that the final outcome, the organism itself, is the product of the integration of tens of thousands of parameters, each of which can be individually set, but that all contribute to the whole.
In essence, this is the objective of the field of evo-devo: to take the parts list that we’re handed by molecular genetics and genomics, to figure out what the Great Formula for an organism is — that is, the relationships between the genetic parts during the development of the embryo — and decipher the permutations possible when the variables are modified by genetic/evolutionary events. We know what the parts are, and we know that many of the parts seem to be held in common from species to species; the next job is to figure out the assembly instructions, how they produced a functional form, and how those rules vary in different species. Cuvier and the scientific anatomists and physiologists of the 19th century could say that the laws of correlation exist, and what we’re trying to do now in the 21st is work through system by system and identify what the laws of correlation are, and the molecular mechanisms underlying their operation.
This is a long introduction to a lovely paper that demonstrates the existence of a developmental rule, a law of correlation, in an evolutionarily significant process. I’m going to let you cogitate over the concepts for a little while, and later I’ll summarize the details.