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What are the views that modern biologist have on Correlation of growth? I need some clarification on statement that I'm gonna mention below as well that is it reason to that as mentioned here: "The gradually increasing perfection of the human hand, and the commensurate adaptation of the feet for erect gait, have undoubtedly, by virtue of such correlation, reacted on other parts of the organism."

Quote mentioned above can be found here: Can be found here

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[...] correlation of growth, as Darwin called it. This law states that the specialised forms of separate parts of an organic being are always bound up with certain forms of other parts that apparently have no connection with them. Thus all animals that have red blood cells without cell nuclei, and in which the head is attached to the first vertebra by means of a double articulation (condyles), also without exception possess lacteal glands for suckling their young. Similarly, cloven hoofs in mammals are regularly associated with the possession of a multiple stomach for rumination. Changes in certain forms involve changes in the form of other parts of the body, although we cannot explain the connection. Perfectly white cats with blue eyes are always, or almost always, deaf. The gradually increasing perfection of the human hand, and the commensurate adaptation of the feet for erect gait, have undoubtedly, by virtue of such correlation, reacted on other parts of the organism. However, this action has not as yet been sufficiently investigated for us to be able to do more here than to state the fact in general terms.

Much more important is the direct, demonstrable influence of the development of the hand on the rest of the organism. It has already been noted that our simian ancestors were gregarious; it is obviously impossible to seek the derivation of man, the most social of all animals, from non-gregarious immediate ancestors. Mastery over nature began with the development of the hand, with labour, and widened man’s horizon at every new advance. He was continually discovering new, hitherto unknown properties in natural objects. On the other hand, the development of labour necessarily helped to bring the members of society closer together by increasing cases of mutual support and joint activity, and by making clear the advantage of this joint activity to each individual. In short, men in the making arrived at the point where they had something to say to each other. Necessity created the organ; the undeveloped larynx of the ape was slowly but surely transformed by modulation to produce constantly more developed modulation, and the organs of the mouth gradually learned to pronounce one articulate sound after another.

First, the term correlation of growth is not often or frequently or widely used in biology. In my career I've know of no reference to it. That's why I preface with a comprehensive quotation to give readers a context of what is meant!

Second: if by correlation of growth, you mean that some trait is concomitant with another trait simply out of evolutionary contingency, then absolutely yes, modern biology would very much support this "correlation". However, it is stated above as a "law", and there exist countless instances in which traits are lost or gained in evolution. Mutations may produce gain- or loss-of function enzymes (genes), for instance. These enzymes possess chemistries hitherto unknown by the ancestral enzymes (genes) and so would violate a correlation with all other traits. It sounds more like a broad pattern or principle of thought than a biological law.

However, the idea that specialized traits are bundled together into organisms, and that organisms may only propagate into the future as lineages, is absolutely in line with modern biology. Thus, the expectation that unconnected traits, such as number of toes and number of ears, co-exist in a lineage for a while, is sound. It is also a little trivial too. At least for the vast majority of cases, in mammals*.

*Plasmid horizontal gene transfer in bacteria and other such things, again, would violate the notion that traits are "bundled" strictly within organisms and are only shared by descent... so I would rather mention that it'd be wiser not to think of it as law but rather as a matter of guiding principle.

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