After publishing a respected three-volume work Flore française, he gained membership into the French Academy of Sciences in 1779, with the help of Georges-Louis Leclerc, Comte de Buffon. Lamarck became involved in the Jardin des Plantes and was appointed to the Chair of Botany in 1788. When the Muséum national d'histoire naturelle was founded in 1793, Lamarck was appointed as a professor of zoology. In 1801, he published Système des animaux sans vertèbres, a major work on the classifications of invertebrates. In an 1802 publication, he became one of the first to use the term biology in its modern sense. Lamarck continued his work as a premier authority on invertebrate zoology.
In the modern era, Lamarck is remembered primarily for a theory of "inheritance of acquired characters", called "soft inheritance" or Lamarckism. However, his descriptions of soft inheritance were, in fact, reflections of the folk wisdom of the time, accepted by most natural historians (including Charles Darwin in his Origin of Species). Lamarck's contribution to evolutionary theory consisted of the first truly cohesive theory of evolution, in which an alchemical complexifying force drove organisms up a ladder of complexity, and a second environmental force adapted them to local environments through "use and disuse" of characteristics, differentiating them from other organisms.
Lamarck was born in Bazentin, Picardie, northern France, as the eleventh child in an impoverished aristocratic family. Male members of the Lamarck family had traditionally served in the French army. Lamarck's eldest brother was killed in combat at the Siege of Bergen-op-Zoom, and two other brothers were still in service when Lamarck was in his teenage years. Yielding to the wishes of his father, Lamarck enrolled in a Jesuit college in Amiens in the late 1750s. After his father died in 1760, Lamarck bought himself a horse, and rode across the country to join the French army, which was in Germany at the time. Lamarck showed great physical courage on the battlefield in the Pomeranian War with Prussia, and he was even nominated for the lieutenancy. Lamarck's company was left exposed to the direct artillery fire of their enemies, and was quickly reduced to just fourteen men - with no officers. One of the men suggested that the puny, seventeen year-old volunteer should assume command and order a withdrawal from the field; but although Lamarck accepted command, he insisted they remain where they had been posted until relieved. When their colonel reached the remains of their company, this display of courage and loyalty impressed him so much that Lamarck was promoted to officer on the spot. However, when one of his comrades playfully lifted him by the head, he sustained an inflammation in the lymphatic glands of the neck, and he was sent to Paris to receive treatment. He underwent a complicated operation, and continued his treatment for a year. He was awarded a commission and settled at his post in Monaco. It was there that he encountered Traité des plantes usuelles, a botany book by James Francis Chomel.
With a reduced pension of only 400 francs a year, Lamarck resolved to pursue a profession. He attempted to study medicine, and supported himself by working in a bank office. Lamarck studied medicine for four years, but gave it up under his elder brother's persuasion. He was interested in botany, especially after his visits to the Jardin du Roi, and he became a student under Bernard de Jussieu, a notable French naturalist. Under Jussieu, Lamarck spent ten years studying French flora. After his studies, in 1778, he published some of his observations and results in a three-volume work, entitled Flore française. Lamarck's work was respected by many scholars, and it launched him into prominence in French science. On August 8, 1778, Lamarck married Marie Anne Rosalie Delaporte. Georges-Louis Leclerc, Comte de Buffon, one of the top French scientists of the day, mentored Lamarck, and helped him gain membership to the French Academy of Sciences in 1779 and a commission as a Royal Botanist in 1781, in which he traveled to foreign botanical gardens and museums. Lamarck's first son, André, was born on April 22, 1781, and he made colleague André Thouin the child's godfather.
In his two years of travel, Lamarck collected rare plants that were not available in the Royal Garden, and also other objects of natural history, such as minerals and ores, that were not found in French museums. On January 7, 1786, his second son, Antoine, was born, and Lamarck chose Antoine Laurent de Jussieu, Bernard de Jussieu's nephew, as the boy's godfather. On April 21 of the following year, Charles Rene, Lamarck's third son, was born. René Louiche Desfontaines, a professor of botany at the Royal Garden, was the boy's godfather, and Lamarck's elder sister, Marie Charlotte Pelagie De Monet was the godmother. In 1788, Buffon's successor at the position of Intendant of the Royal Garden, Charles-Claude Flahaut de la Billaderie, comte d'Angiviller, created a position for Lamarck, with a yearly salary of 1,000 francs, as the keeper of the herbarium of the Royal Garden.
In 1790, at the height of the French Revolution, Lamarck changed the name of the Royal Garden from Jardin du Roi to Jardin des Plantes, a name that did not imply such a close association with King Louis XVI. Lamarck had worked as the keeper of the herbarium for five years before he was appointed curator and professor of invertebrate zoology at the Muséum national d'histoire naturelle in 1793. During his time at the herbarium, Lamarck's wife gave birth to three more children before dying on September 27, 1792. With the official title of "Professeur d’Histoire naturelle des Insectes et des Vers", Lamarck received a salary of nearly 2,500 francs per year. The following year on October 9, he married Charlotte Reverdy, who was thirty years his junior. On September 26, 1794, Lamarck was appointed to serve as secretary of the assembly of professors for the museum for a period of one year. In 1797, his second wife Charlotte died, and he remarried Julie Mallet the following year, who later died in 1819.
In his first six years as professor, Lamarck published only one paper, in 1798, on the influence of the moon on the earth's atmosphere. Lamarck began as an essentialist who believed species were unchanging; however, after working on the molluscs of the Paris Basin, he grew convinced that transmutation or change in the nature of a species occurred over time. He set out to develop an explanation, and on 11 May 1800 (the 21st day of Floreal, Year VIII, in the revolutionary timescale used in France at the time), he presented a lecture at the Muséum national d'histoire naturelle in which he first outlined his newly developing ideas about evolution.
In 1801, he published Système des Animaux sans Vertebres, a major work on the classification of invertebrates. In the work, he introduced definitions of natural groups among invertebrates. He categorized echinoderms, arachnids, crustaceans and annelids, which he separated from the old taxon for worms known as Vermes. Lamarck was the first to separate arachnids from insects in classification, and he moved crustaceans into a separate class from insects. In the class Crustaces, Lamarck proposed two orders. The first group was Crustaces pediocles, classified by the animals' two distinct stalked eyes. This categorization included crabs, shrimps and hermit crabs. The second group Crustaces sessiliocles were classified by the animals' distinct pair of eyes, or single, sessile eyes. Members of this order included amphipods, isopods, cyclops, cladocerans, xiphosurida and amymones.
In 1802 Lamarck published Hydrogéologie, and became one of the first to use the term biology in its modern sense. In Hydrogéologie, Lamarck advocated a steady-state geology based on a strict uniformitarianism. He argued that global currents tended to flow from east to west, and continents eroded on their eastern borders, with the material carried across to be deposited on the western borders. Thus, the Earth's continents marched steadily westward around the globe.
That year, he also published Recherches sur l'Organisation des Corps Vivans, in which he drew out his theory on evolution. He believed that all life was organized in a vertical chain, with gradation between the lowest forms and the highest forms of life, thus demonstrating a path to progressive developments in nature.
During his lifetime he became controversial, attacking the chemistry proposed by Lavoisier and criticising palaeontologist Georges Cuvier’s anti-evolutionary stance. After his death, Cuvier used the forum of a eulogy in an attempt to discredit Lamarck's scientific beliefs, painting a picture of Lamarck as the equivalent of a philosopher or poet (in fact, Lamarck was a strict materialist).
Lamarck died in Paris on December 18, 1829. When he died, his family were so poor they had to apply to the Academie for financial assistance. Lamarck's books and the contents of his home were sold at auction, and he was buried in a temporary lime-pit.
Lamarck stressed two main themes in his biological work. The first was that the environment gives rise to changes in animals. He cited examples of blindness in moles, the presence of teeth in mammals and the absence of teeth in birds as evidence of this principle. The second principle was that life was structured in an orderly manner and that many different parts of all bodies make it possible for the organic movements of animals.
Although he was not the first thinker to advocate organic evolution, he was the first to develop a truly coherent evolutionary theory. He outlined his theories regarding evolution first in his Floreal lecture of 1800, and then in three later published works:
Lamarck employed several mechanisms as drivers of evolution, drawn from the common knowledge of his day and from his own belief in chemistry pre-Lavoisier. He used these mechanisms to explain the two forces he saw as comprising evolution; a force driving animals from simple to complex forms, and a force adapting animals to their local environments and differentiating them from each other. He believed that these forces must be explained as a necessary consequence of basic physical principles, favoring a materialistic attitude toward biology.
Lamarck ran against the modern chemistry promoted by Lavoisier (whose ideas he regarded with disdain), preferring to embrace a more traditional alchemical view of the elements as influenced primarily by earth, air, fire and water. He asserted that the natural movements of fluids in living organisms drove them toward ever greater levels of complexity:
He argued that organisms thus moved from simple to complex in a steady, predictable way based on the fundamental physical principles of alchemy. In this view, simple organisms never disappeared because they were constantly being created by spontaneous generation in what has been described as a 'steady-state biology'. Lamarck saw spontaneous generation as being ongoing, with the simple organisms thus created being transmuted over time becoming more complex. He is sometimes regarded as believing in a teleological (goal-oriented) process where organisms became more perfect as they evolved, though as a materialist, he emphasized that these forces must originate necessarily from underlying physical principles.
Lamarck argued that this adaptive force was powered by the interaction of organisms with their environment, by the use and disuse of characters:
These characters were then inherited, according to the common belief of the day, in what is known as "soft inheritance" (nowadays erroneously called Lamarckism):
Lamarck constructed what may be the first comprehensive theoretical framework of organic evolution. Stephen Jay Gould argues that Lamarck was the "primary evolutionary theorist", in that his ideas and the way in which he structured his theory set the tone for much of the subsequent thinking in evolutionary biology, through to the present day.
Lamarck is usually remembered for his belief in the inheritance of acquired characteristics, and the "use and disuse" model by which organisms developed their characteristics. Lamarck incorporated this belief into his theory of evolution, along with other more common beliefs of the time, such as spontaneous generation.
The inheritance of acquired characteristics (also called the theory of adaptation or "soft inheritance") was rejected by August Weismann when he developed a theory of inheritance in which "germ-plasm" (the hereditary material passed from parents to offspring) remained separate and distinct from "soma" (the material composing the body of an organism); thus nothing which happens to the soma may be passed on with the germ-plasm. This model underlies the modern understanding of inheritance. Weismann is famous for an experiment in which he cut the tails off mice, demonstrating that the injury was not passed on to the offspring; but historians of science such as Stephen Jay Gould argue that this experiment had far less effect on the acceptance of Lamarckism than Weismann's more comprehensive theoretical framework (Believers in soft inheritance did not count injury or mutilation as a true acquired characteristic: only those which were initiated by the animal's own needs, that were beneficial, were expected to be passed on). This Lamarckian view is consistent with Charles Darwin's theory of natural selection.
Lamarckism is used as an analogy to describe the action of other "evolutionary" concepts in the modern era. For example, the memetic theory of cultural evolution is sometimes described as a form of "Lamarckian" inheritance of non-genetic traits.