Final Answer:
The 18th-century thinker who believed that living forms change in response to the environment and often argued with Linnaeus about the fixity of species was Jean-Baptiste Lamarck.
Step-by-step explanation:
Jean-Baptiste Lamarck was a prominent French naturalist who proposed the theory of the inheritance of acquired characteristics, known as Lamarckism. In the 18th century, Lamarck argued against the prevailing view held by Carl Linnaeus that species were fixed and unchanging. Lamarck believed that organisms could change over time in response to their environment, and these acquired traits could be passed on to future generations.
Lamarck's theory suggested that if an organism used or failed to use a particular organ, it would either develop or deteriorate over time. He proposed that these changes would be inherited by offspring, leading to the evolution of species. Lamarck's ideas laid the groundwork for later evolutionary theories, although his specific mechanisms of evolution were largely rejected in favor of Darwinian natural selection.
The debates between Lamarck and Linnaeus reflected the broader intellectual shifts occurring in the 18th century, challenging the fixity of species and contributing to the development of evolutionary thought. Despite the eventual acceptance of Darwin's natural selection, Lamarck's ideas played a crucial role in shaping early discussions on the dynamic nature of life forms and their adaptation to changing environments.