But the main idea of Lamarckian evolutionism was what became known as the inheritance of acquired characters. This described the means by which the structure of an organism altered over generations. His theory stated that change occurred because an animal passed on to its offspring physiological changes it had undergone in its own lifetime, and those changes came about by its responding to its survival needs. The long legs and webbed feet of wading birds, for example, arose when those animals' ancestors responded to a need to feed on fish. In their attempt to wade in deeper and yet still keep their bodies dry, they would unconsciously adopt the habit of stretching their legs to their full extent, making them minutely longer in the process. This trait would be passed on to the next generation, who would in turn stretch their legs, until over many generations the wading birds' legs became long enough for the wading bird. As the longer-legged birds waded deeper, it would be the habit of spreading the toes in order not to sink into the offshore mud, stretching the skin in-between, which would eventually give rise to webbed feet. On the other hand, the disuse of an organ would cause it to wither and disappear, which explained how snakes may lost their legs.…
