It states that changes in environmet bring about bodily change sin individuals by their inner urge. Such changes are developed by use and are handed over to the next generation. Thus, his theory centres around the idea of use and disuse of parts and inheritence of acquired characters. He gave his theroy in his book "Phylosophie Zoologique".
He considered that every organism has the wish, desire, will and needs to increase in size and complexity. When the environment changes, there is an alteration in the needs of organism. Thus, its not the environment that causes changes but the urge in organism that shapes it. The change in environment necessitates new habits and activities. Thus some organs are made more effecient while other are rendered useless. Tese are passed on then to the next generation and in due time new species are born.
His theory is based on four ideas-
1. Theory of growth-
The eternal force of life tends to increase the size of an organism by growth in organs and systems. Thus the internal force is capable of forming new organs and increasing the complexity of lifeforms. Ex- A seed grows to be a giant tree and a zygote develops to be a large creature.
2. Theory of environmental pressure and spontaneous formation of organs.
Changes in invironment inititate the desire in organism to develop new organs to meed the challenges of environment.
3. Theory of use and disuse.
If an organ is used continuously, it will develop while one in disuse will de-generate.
4. Theory of inheritence of acquited character.
All changes acquired in one generation are passed on to the next generation.
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