Lamarckism is a hypothesis of evolution based on the notion that physical changes in organisms during their lifetime, such as increased development of an organ or a portion as a result of increasing use, can be passed down to their offspring. The idea, developed in 1809 by the French naturalist Jean-Baptiste Lamarck, impacted evolutionary thought for the rest of the nineteenth century. Most geneticists dismissed Lamarckism in the 1930s, but some of its theories persisted in the Soviet Union until the mid-twentieth century.

Propositions of Lamarckism theory:

• Because of inherent forces of life, living organisms and their parts tend to grow in size throughout time.
• In response to new demands, organisms’ bodies generate new organs. For example, with the evolution of the horse, the plantigrade foot gave way to the unguligrade foot. The horse’s ancestor lived on soft ground, where the plantigrade foot sufficed. When the soft ground in the rainforest gave way to dry hard land, the foot altered to an unguligrade kind that was better suited to running on hard ground.
• When an organ is used continually and consistently, it becomes highly developed, whereas overuse leads to deterioration.

Some examples are:

(i) By extending the skin between the toes, aquatic birds produce webbed feet.

(ii) Limbs are missing in snakes, Proteus, and certain burrowing creatures because they are useless for crawling and are an impediment to mobility.

(iii) The giraffe’s long neck evolved as a result of frequent stretching to reach food.

(iv) The muscles of the external ear, or pinna, are diminished in humans but functional and well developed in animals such as dogs and rabbits to gather sound waves.

• Changes acquired throughout an individual’s lifetime are inherited by their children.

Criticism of Lamarckism theory:

• Though there has been evidence of an increase in size in various forms, there have also been cases of a decrease in size. Primitive trees, for example, are large, whereas shrubs, herbs, and grasses that emerged later are lower in size.
• If new organs grow in response to new needs, man should have developed wings by now.
• Changes gained by an organism during its lifetime are not inherited by its children. For example, if a guy loses an arm in battle, he does not have armless children. According to August Weismann, somatic alterations gained during an organism’s existence are not heritable, whereas changes in the germplasm or reproductive cells are.
• Lamarck’s idea was also debunked by Mendel’s law of inheritance.

Neo-Lamarckism:

Theories incorporating the inheritance of features acquired during an organism’s existence were among the most prominent choices. Neo-Lamarckians were scientists who believed that such Lamarckian mechanisms were the key to evolution. They included British botanist George Henslow (1835–1925), who studied the effects of environmental stress on plant growth in the hope that such environmental variation could explain much of plant evolution, and American entomologist Alpheus Spring Packard, Jr., who studied blind animals living in caves and wrote a book about Lamarck and his work in 1901. 

Palaeontologists such as Edward Drinker Cope and Alpheus Hyatt were also included, who noticed that the fossil record revealed systematic, nearly linear patterns of growth that they felt were best explained by Lamarckian mechanisms rather than natural selection. Some people, like Cope and Darwin critic Samuel Butler, believed that inheriting learned qualities would allow creatures to determine their evolution, because organisms that acquired new habits would modify the use patterns of their organs, triggering Lamarckian evolution. They thought this was more intellectually sound than Darwin’s theory of random variation influenced by selective pressures. 

Lamarckism also appealed to individuals who saw evolution as an intrinsically progressive process, such as the philosopher Herbert Spencer and the German anatomist Ernst Haeckel. Theodor Eimer, a German naturalist, blended Lamarckism with notions concerning orthogenesis, or the belief that evolution is guided toward a goal.

Lamarckism went out of favour with the advent of the contemporary synthesis of evolutionary theory and a lack of evidence for a mechanism for acquiring and passing on new features, or even their heritability. Unlike neo-Darwinism, neo-Lamarckism is a loose confederation of primarily heterodox hypotheses and mechanisms that evolved following Lamarck’s death, rather than a cohesive body of theoretical work.

Conclusion:

Many studies sought to find evidence for Lamarckian inheritance beginning in the 1860s, but all of these have been explained away, either by alternative mechanisms such as genetic contamination or by deception. August Weismann’s experiment, which was thought to be definitive at the time, is now thought to have failed to disprove Lamarckism since it did not address use and disuse. Later, Mendelian genetics substituted the concept of acquired trait inheritance, finally leading to the formation of the modern synthesis and the abolition of Lamarckism in biology. Despite this, Lamarckism has remained popular. Epigenetics, genetics, and somatic hypermutation studies have highlighted the possibility of inheriting traits acquired by the previous generation. The labelling of these findings as Lamarckism has been contested.