Theory of evolution that stresses the inheritance of acquired characteristics as the mechanism of organic change. Over time somewhat different species of Lamarckianism have emerged, but at a basic level all share the belief that the physical changes, structural enhancements, and sometimes even mental qualities achieved by an organism in its lifetime can directly effect the genetic makeup of subsequent generations. The concept was first put forth in a rigorous scientific fashion by French naturalist Jean Baptiste Lamarck in the early 19th century.
Lamarck’s belief in the inheritance of acquired characteristics was actually part of a more comprehensive theory of natural history and organic development. Although he never used the term evolution, his ideas were one of the earliest systematic attempts to explain the development of complex organisms from simple ones. In his theory, Lamarck accounted for the origin of life via spontaneous generation. Once the simplest organisms (both plants and animals) had been produced, environmental influences and time affected them in such a way as to produce the variety of complex organisms now found in the world.