Lamarckians are traditionally fond of calluses, so let us use that example. Our hypothetical bank clerk had soft, pampered hands except for a hard callus on the middle finger of his right hand, his writing finger. If generations of his descendants all write a great deal, the Lamarckian expects that genes controlling the development of skin in that region will be altered in such a way that babies come to be born with the appropriate finger already hardened. If the genes were a blueprint this would be easy. There would be a gene 'for' each square millimetre (or appropriate small unit) of skin. The whole surface of the skin of an adult bank clerk would be 'scanned', the hardness of each square millimetre carefully recorded and fed back into the genes 'for' that particular square millimetre, in particular the appropriate genes in his sperms.
But the genes are not a blueprint. There is no sense in which there is a gene 'for' each square millimetre. There is no sense in which the adult body could be scanned and its description fed back into the genes. The 'coordinates' of a callus could not be 'looked up' in the genetic record and the 'appropriate' genes altered. Embryonic development is a process, in which all working genes participate; a process which, if correctly followed in the forward direction, will result in an adult body; but it is a process that is inherently, by its very nature, irreversible. The inheritance of acquired characteristics not only doesn't happen: it couldn't happen in any life-form whose embryonic development is epigenetic rather than preformationistic. Any biologist that advocates Lamarckism is, though he may be shocked to hear it, implicitly advocating an atomistic, deterministic, reductionistic embryology. I didn't want to burden the general reader with that little string of pretentious jargon words: I just couldn't resist the irony, for the biologists who come closest to sympathising with Lamarckism today also happen to be particularly fond of using those same cant words in criticising others.
