According to this theory, individuals lose characteristics they do not require (or use) and develop characteristics that are useful. These characteristics or traits are caused by changes or mutations in the organism that are directed towards some objective so the organism would become better adapted to a particular situation or to its environment, and the traits acquired or learned during its lifetime can be passed on to its offspring which would also be possessing these traits.
However, this theory is no longer widely accepted like before. For instance, according to this theory, the giraffes evolve into having long necks because of their stretching to reach for the leaves on tall trees, and this trait had been passed on to its offspring. But other leaf-eating animals with short necks would not need a long neck to survive. A sheep does not need longer neck as it can eat leaves from grass. This indicates the giraffe's neck is long not because of the result of direct mutation nor because it is a requisite for survival, but because it is caused by random mutation.
Word origin: after the French biologist Jean-Baptiste Lamarck, who incorporated the action of soft inheritance into his evolutionary theories.
See also: direct mutation, soft inheritance.