Inheritance of acquired characteristics

Many researchers from the 1860s onwards attempted to find evidence for the theory, but these have all been explained away either by other mechanisms such as genetic contamination, or as fraud. On the other hand, August Weismann's experiment is now considered to have failed to disprove Lamarckism as it did not address use and disuse. Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern synthesis, and the general abandonment of Lamarckism in biology. Despite this, interest in Lamarckism has continued. Studies in the field of epigenetics, genetics and somatic hypermutation have highlighted the possible inheritance of traits acquired by the previous generation. The characterization of these findings as Lamarckism has been disputed. The inheritance of the hologenome, consisting of the genomes of all an organism's symbiotic microbes as well as its own genome, is also somewhat Lamarckian in effect, though entirely Darwinian in its mechanisms. The inheritance of acquired characteristics was proposed in ancient times, and remained a current idea for many centuries. The historian of science Conway Zirkle wrote in 1935 that: Zirkle noted that Hippocrates described pangenesis, the theory that what is inherited derives from the whole body of the parent, whereas Aristotle thought it impossible; but that all the same, Aristotle implicitly agreed to the inheritance of acquired characteristics, giving the example of the inheritance of a scar, or of blindness, though noting that children do not always resemble their parents. Zirkle recorded that Pliny the Elder thought much the same. Zirkle also pointed out that stories involving the idea of inheritance of acquired characteristics appear numerous times in ancient mythology and the Bible, and persisted through to Rudyard Kipling's Just So Stories. Erasmus Darwin's Zoonomia (c. 1795) suggested that warm-blooded animals develop from 'one living filament... with the power of acquiring new parts' in response to stimuli, with each round of 'improvements' being inherited by successive generations. Charles Darwin's On the Origin of Species proposed natural selection as the main mechanism for development of species, but did not rule out a variant of Lamarckism as a supplementary mechanism. Darwin called this pangenesis, and explained it in the final chapter of his book The Variation of Animals and Plants under Domestication (1868), after describing numerous examples to demonstrate what he considered to be the inheritance of acquired characteristics. Pangenesis, which he emphasised was a hypothesis, was based on the idea that somatic cells would, in response to environmental stimulation (use and disuse), throw off 'gemmules' or 'pangenes' which travelled around the body, though not necessarily in the bloodstream. These pangenes were microscopic particles that supposedly contained information about the characteristics of their parent cell, and Darwin believed that they eventually accumulated in the germ cells where they could pass on to the next generation the newly acquired characteristics of the parents. Darwin's half-cousin, Francis Galton, carried out experiments on rabbits, with Darwin's cooperation, in which he transfused the blood of one variety of rabbit into another variety in the expectation that its offspring would show some characteristics of the first. They did not, and Galton declared that he had disproved Darwin's hypothesis of pangenesis, but Darwin objected, in a letter to the scientific journal Nature, that he had done nothing of the sort, since he had never mentioned blood in his writings. He pointed out that he regarded pangenesis as occurring in protozoa and plants, which have no blood, as well as in animals. Between 1800 and 1830, Lamarck proposed a systematic theoretical framework for understanding evolution. He saw evolution as comprising four laws: In 1830, in an aside from his evolutionary framework, Lamarck briefly mentioned two traditional ideas in his discussion of heredity, in his day considered to be generally true. The first was the idea of use versus disuse; he theorized that individuals lose characteristics they do not require, or use, and develop characteristics that are useful. The second was to argue that the acquired traits were heritable. He gave as an imagined illustration the idea that when giraffes stretch their necks to reach leaves high in trees, they would strengthen and gradually lengthen their necks. These giraffes would then have offspring with slightly longer necks. In the same way, he argued, a blacksmith, through his work, strengthens the muscles in his arms, and thus his sons would have similar muscular development when they mature. Lamarck stated the following two laws: