Jean Baptiste Lamarck (1744-1829) trained as a botanist before becoming a zoologist. Though his contributions covered various topics, he is most recognised for being the first to argue against species immutability and for his outstanding thesis, Lamarckism. Evolution, according to Lamarck, was a never-ending quest for perfection on the part of God's creatures. He believed in Aristotle's concept of scale and used it to build his theory of evolution.
Life, he claims, has always existed in its most basic form. It then unfolded across many generations, achieving complexity owing to its intrinsic drive. As a result, it progressed to higher levels of the organisation through a series of intermediate gradations.
Lamarck understood that the physical world changed over time and claimed that organisms must adapt to live. How did this happen? According to Lamarck, organisms and their constituent elements, such as organs, tended to grow in size indefinitely. To accommodate the need, new organs were created. Third, organs that were constantly utilised developed; on the other hand, organs that were not used degenerated.
Thus, organisms respond to environmental changes by gradually altering their physiological structure. Every such modification, no matter how little, was handed down to the descendants. Evolution is the cumulative result of such inheritance of acquired characteristics over long periods. This kind of view is known as Lamarckism.
The most critical pre-Darwinian evolutionary idea, without a doubt, was offered by the French scientist Jean Baptist-Lamarck, a professor of Zoology in Paris. In 1809, he presented his theory of organic evolution in the book Philosophique - Zoologique, which received widespread attention. Lamarck's hypothesis assumed that species differed because they had distinct requirements. Some organs and appendages were utilised more than others due to their particular demands.
His evolutionary thinking was founded on four assumptions −
The Inner Urge of Organisms − Organisms tend to develop and expand in size. According to Lamarck, this size, form, and structure development results from an inner need for life.
New Needs Arise due to the Ever-Changing Environment − Lamarck saw that the surroundings were not constant. It was constantly changing, which resulted in establishing a new environment. The creatures then attempted to adapt to their new habitat in order to live. As a result, structural alterations and behavioural changes occurred in them. In reality, organisms acquire adaptive structural alterations to adapt to their new environment.
Use and Disuse of Organs − Lamarck believed that when the environment changed, some organs/appendages of the body were used more frequently than others. Organs and appendages utilised more frequently grow and develop more effectively, whereas those used less frequently in the altered environment become vestigial or diminished.
Inheritance of Acquired Characters − Lamarck recognised that organisms evolve new adaptive characteristics in response to environmental changes during their lives. He referred to them as acquired characteristics. These acquired characteristics are not found in its predecessor. These acquired characteristics, according to Lamarck, are passed on to the following generation, resulting in morphological, physiological, and behavioural changes in a species.
Essentially, Lamarck's theory is divided into two parts −
New structures emerge due to an organism's intrinsic desire.
These new structures are acquired in response to a need and handed on to future generations.
In support of his arguments or ideas, Lamarck presented several instances from diverse groups of animals. We will look at two famous instances in this section.
Lamarck's hypothesised theory of evolution was heavily criticised. Several evolutionists were required to defend their points of view against Lamarck's theory of evolution. Darlington considers Lamark's notion to be an evergreen superstition. Many aims were presented in opposition to Lamarkism.
Tendency to Increase in Size − The first law of Lamark asserts that all creatures tend to grow in size. It does not have widespread acceptance. Some poriferans are substantially larger than coelenterates. Many fish are more significant than amphibians, reptiles, birds, and mammals.
Inheritance of Acquired Characters − August Weisman (1904) refuted Lamarck's thesis that whatever is gained during a person's lifetime is handed down to the next generation. He severed rats' tails for numerous generations (about 22 generations) and allowed them to reproduce. Weisman discovered that there was never a rat without a tail in any generation. Only the traits that impact germ cells are handed on to the following generation. Any alterations in somatic cells are not transmitted to the following generation. This was referred to as the "theory of germplasm."
Use and Disuse of Organs − Lamarck's use and disuse hypothesis was also rejected since it caused alterations in somatic cells that could not be passed down to progeny. Although organs such as the heart and eye are constantly employed, no changes in their size or musculature have been recorded thus far.
Formation of New Organ at Will − Evolutionists contended that if Lamarck's idea that "new organs develop because of an organism's need and inner urge" was correct, then people who have always longed to fly could not create wings until now.
Evolution, according to Lamarck, occurred linearly. Straight-line evolution occurs when species evolve predictably, which may lead to extinction.
Despite significant criticism of Lamarck's thesis, a few followers sought to revitalise it by offering their views. Neo-Lamarckism refers to Lamarck's improved theory. Although the transmission of the effects of use and disuse has not been shown, it is clear that possibilities exist, according to Neo-Lamarckism. Neo-Lamarckians conducted a variety of studies to demonstrate that children transfer acquired characteristics. Griffith and Detleoson raised the rats on a revolving table for several months.
The rats were so used to the shift that when the rotation was switched, the animals experienced dizziness and a change in physiological state. When these rats bred among themselves, their progeny were dizzy when the spinning was halted. In another experiment, Guyer and Smith smashed a rabbit lens and put it in a fowl to create antibodies. The fowl antiserum was then injected into a pregnant rabbit, and it was discovered that some of the offspring had damaged eyes. This personality trait was passed down down the generations.
T.H. Morgan and Cope hypothesised that all modifications an organism acquires throughout its lifetime are heritable if integrated with the germplasm. That accumulation of these changes generation after generation culminates in the emergence of new species.
Neo-Lamarckism emphasises the direct influence of changing environments on organisms.
Only those alterations that affect germ cells are passed on to the following generation.
Although Lamarck's beliefs on evolution were incorrect, he deserves credit for being the first to develop a cohesive theory of evolution.
Lamarckism argued that evolution was a never-ending quest for perfection on the part of God's creatures, based on four assumptions: the Inner Urge of Organisms, the Inner Need for Life, the Ever-Changing Environment, and the Inheritance of acquired characteristics. Lamarck's theory of evolution states that organisms evolve new adaptive characteristics in response to environmental changes, which are passed on to the following generation, resulting in morphological, physiological, and behavioural changes in a species.
Neo-Lamarckism emphasizes the direct influence of changing environments on organisms, and believes that only those alterations that affect germ cells are passed on to the following generation.
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