Lamarck believed that organisms change because changes in the environment cause changes in the needs of organisms. He suggested that organisms alter their behavior because of this changed need. For example if the environment becomes dry and vegetation will only be available high above the ground, animals will use their necks more to reach out for the leaves. He believed that this increased use will result in tissue change in the animal’s necks. Lamarck also proposed that animal’s offspring will inherit this changed tissue. For example, he believed that the necks of giraffes went through a gradual change over generations getting longer with each generation. Lamarck believed that the process of evolution was not driven by chance, rather with each generation more complex and more perfect forms of life appeared. Because of this Lamarck did not believe in extinction. For him species that disappeared did so because they evolved into different species.

In support of his theory Lamarck mentioned the possibility of natural selection and cited evidence such as :

The great variety of plants and animals that were produced by selective breeding under human cultivation.

The presence of body parts in some animals that are not used but show similarities to ancestors.

Some embryonic structures exist during development that are not present in adults.

Lamarck’s theory of evolution came to be known as The Theory of Inheritance of Acquired Traits. It is now often used in a rather derogatory sense.

Lamarckian inheritance at least, in the sense Lamarck intended, is in conflict with the findings of genetics and has now been largely abandoned. But until the rediscovery of Mendel’s Law at the beginning of the 20^th century, no one understood the mechanism of heredity, and Lamarckian inheritance was a perfectly reasonable hypothesis accepted till then.