Lamarckism (or Lamarckian evolution) is the once widely accepted idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring (also known as based on heritability of acquired characteristics or "soft inheritance"). It is named for the French biologist Jean-Baptiste Lamarck (1744–1829), who incorporated the action of soft inheritance into his evolutionary theories and is often incorrectly cited as the founder of soft inheritance. It proposed that individual efforts during the lifetime of the organisms were the main mechanism driving species to adaptation, as they supposedly would acquire adaptive changes and pass them on to offspring.

After publication of Charles Darwin's theory of natural selection, the importance of individual efforts in the generation of adaptation was considerably diminished. Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern evolutionary synthesis, and the general abandonment of the Lamarckian theory of evolution in biology. In a wider context, soft inheritance is of use when examining the evolution of cultures and ideas, and is related to the theory of Memetics.
While enormously popular during the early 19th century as an explanation for the complexity observed in living systems, the relevance of soft inheritance within the scientific community dwindled following the theories of August Weismann and the formation of the modern evolutionary synthesis.
History

Between 1794 and 1796 Erasmus Darwin wrote Zoönomia suggesting "that all warm-blooded animals have arisen 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. Subsequently Jean-Baptiste Lamarck repeated in his Philosophie Zoologique of 1809 the folk wisdom that characteristics which were "needed" were acquired (or diminished) during the lifetime of an organism then passed on to the offspring. He incorporated this mechanism into his thoughts on evolution, seeing it as resulting in the adaptation of life to local environments.
Lamarck founded a school of French Transformationism which included Étienne Geoffroy Saint-Hilaire, and which corresponded with a radical British school of comparative anatomy based at the University of Edinburgh which included the surgeon Robert Knox and the anatomist Robert Edmund Grant. Professor Robert Jameson wrote an anonymous paper in 1826 praising "Mr. Lamarck" for explaining how the higher animals had "evolved" from the "simplest worms" – this was the first use of the word "evolved" in a modern sense. As a young student, Charles Darwin was tutored by Grant, and worked with him on marine creatures.
The Vestiges of the Natural History of Creation, authored by Robert Chambers and published anonymously in England in 1844, proposed a theory modelled after Lamarckism, causing political controversy for its radicalism and unorthodoxy, but exciting popular interest and paving the way for Darwin.
Darwin's 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 his Lamarckian hypothesis Pangenesis, and explained it in the final chapter of his book Variation in Plants and Animals under Domestication, 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' 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 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. With the development of the modern synthesis of the theory of evolution and a lack of evidence for either a mechanism or even the heritability of acquired characteristics, Lamarckism largely fell from favor.
In the 1920s, experiments by Paul Kammerer on amphibians, particularly the midwife toad, appeared to find evidence supporting Lamarckism, but his specimens with supposedly-acquired black foot-pads were found to have been tampered with. In The Case of the Midwife Toad Arthur Koestler surmised that the specimens had been faked by a Nazi sympathiser to discredit Kammerer for his political views.
A form of "Lamarckism" was revived in the Soviet Union of the 1930s when Trofim Lysenko promoted Lysenkoism which suited the ideological opposition of Joseph Stalin to Genetics. This ideologically driven research influenced Soviet agricultural policy which in turn was later blamed for crop failures.
Since 1988 certain scientists have produced work proposing that Lamarckism could apply to single celled organisms. The lack of evidence for Lamarckian acquisition in higher order animals is still clung to in certain branches of psychology, as, for example, in the Jungian racial memory.
Neo-Lamarckism is a theory of inheritance based on a modification and extension of Lamarckism, essentially maintaining the principle that genetic changes can be influenced and directed by environmental factors.
Lamarck's theory

The identification of "Lamarckism" with the inheritance of acquired characteristics is regarded by some as an artifact of the subsequent history of evolutionary thought, repeated in textbooks without analysis. Stephen Jay Gould wrote that late 19th century evolutionists "re-read Lamarck, cast aside the guts of it ... and elevated one aspect of the mechanics - inheritance of acquired characters - to a central focus it never had for Lamarck himself. He argued that "the restriction of "Lamarckism" to this relatively small and non-distinctive corner of Lamarck's thought must be labelled as more than a misnomer, and truly a discredit to the memory of a man and his much more comprehensive system. Gould advocated defining "Lamarckism" more broadly, in line with Lamarck's overall evolutionary theory.
Lamarck incorporated two ideas into his theory of evolution, in his day considered to be generally true:

1. Use and disuse – Individuals lose characteristics they do not require (or use) and develop characteristics that are useful.
2. Inheritance of acquired traits – Individuals inherit the traits of their ancestors.

Examples of what is traditionally called "Lamarckism" would include:

• Giraffes stretching their necks to reach leaves high in trees (especially Acacias), strengthen and gradually lengthen their necks. These giraffes have offspring with slightly longer necks (also known as "soft inheritance").
• A blacksmith, through his work, strengthens the muscles in his arms. His sons will have similar muscular development when they mature.

With this in mind, Lamarck has been credited in some textbooks and popular culture with developing two laws:

1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.

In essence, a change in the environment brings about change in "needs" (besoins), resulting in change in behavior, bringing change in organ usage and development, bringing change in form over time — and thus the gradual transmutation of the species.
However, as scientist historians such as Michael Ghiselin and Stephen Jay Gould have pointed out, none of these views were original to Lamarck. On the contrary, Lamarck's contribution was a systematic theoretical framework for understanding evolution. He saw evolution as comprising two processes;

1. Le pouvoir de la vie (a complexifying force) - in which the natural, alchemical movements of fluids would etch out organs from tissues, leading to ever more complex construction regardless of the organ's use or disuse. This would drive organisms from simple to complex forms.
2. L'influence des circonstances (an adaptive force) - in which the use and disuse of characters led organisms to become more adapted to their environment. This would take organisms sideways off the path from simple to complex, specialising them for their environment.

Current views on "Lamarckism"

The argument that instinct in animals is evidence for hereditary knowledge is generally regarded within science as false. Current views suggest that behaviours are more probably passed on through a mechanism called the Baldwin effect. While such a theory might explain the observed diversity of species and the first law is generally true, the main argument against Lamarckism is that experiments simply do not support the second law — purely "acquired traits" do not appear in any meaningful sense to be inherited. For example, a human child must learn how to catch a ball even though his or her parents learned the same feat when they were children. Lamarck’s theories gained initial acceptance because the mechanisms of inheritance were not elucidated until later in the 19th Century, after Lamarck's death.
Several historians have argued that Lamarck's name is linked somewhat unfairly to the theory that has come to bear his name, and that Lamarck deserves credit for being an influential early proponent of the concept of biological evolution, far more than for the mechanism of evolution, in which he simply followed the accepted wisdom of his time. Lamarck died 30 years before the first publication of Charles Darwin's Origin of Species. As science historian Stephen Jay Gould has noted, if Lamarck had been aware of Darwin's proposed mechanism of natural selection, there is no reason to assume he would not have accepted it as a more likely alternative to his "own" mechanism. Note also that Darwin, like Lamarck, lacked a plausible alternative mechanism of inheritance - the particulate nature of inheritance was only to be observed by Gregor Mendel somewhat later, published in 1866. Its importance, although Darwin cited Mendel's paper, was not recognised until the Modern evolutionary synthesis in the early 1900s. An important point in its favour at the time was that Lamarck's theory contained a mechanism describing how variation is maintained, which Darwin’s own theory lacked.
Neo-Lamarckism

Unlike neo-Darwinism, the term neo-Lamarckism refers more to a loose grouping of largely heterodox theories and mechanisms that emerged after Lamarck's time, than to any coherent body of theoretical work.
In the 1920s, Harvard University researcher William McDougall studied the abilities of rats to correctly solve mazes. He found that children of rats that had learned the maze were able to run it faster. The first rats would get it wrong 165 times before being able to run it perfectly each time, but after a few generations it was down to 20. McDougall attributed this to some sort of Lamarckian evolutionary process.
McDougall's results were later shown to be incorrect and caused by poor experimental controls by Oscar Werner Tiegs and Wilfred Eade Agar.
At around the same time, Ivan Pavlov, who was also a Lamarckist, claimed to have observed a similar phenomena in animals being subject to conditioned reflex experiments. He claimed that with each generation, the animals became easier to condition. Neither McDougall or Pavlov suggested a mechanism to explain their observations.