The rise of evolutionary biology - What was biology before Darwin like?
In his famous discussion of the giraffe's neck, Lamarck argued that ancestral giraffes had stretched to reach leaves higher up trees; the exertion caused their necks to grow slightly longer; their longer necks were inherited by their offspring, who thus started life with a propensity to grow even longer necks than their parents. After many generations of neck-stretching, the result is what we can see now.
Lamarck described the process as being driven by the 'striving' of the giraffe, and he often described animals as 'wishing' or 'willing' to change themselves. His theory has therefore sometimes been caricatured as suggesting that evolution happens by the will of the organism. However, the theory does not require any conscious striving on the part of the organism - only some flexibility in individual development and the inheritance of acquired characters.
Lamarck did not invent the idea of the inheritance of acquired characters. It is an ancient idea even discussed by Plato. However, most modern thinking about the role of the process in evolution has been inspired by Lamarck, and the inheritance of acquired characters is now conventionally, if unhistorically, called Lamarckian inheritance.