Life tends to increase the volume of each living body and of all its parts up to a limit determined by its own necessities.
2.

New wants in animals give rise to new organs.
3.

The development of these organs is in proportion to their employment.
4.

New developments may be transmitted to offspring.
His well-known examples to illustrate these views, such as that of successive generations of giraffes lengthening their necks by stretching them to gather high-growing foliage, and of successive generations of kangaroos lengthening and strengthening their hind legs by the necessity of keeping themselves erect while jumping, provoked laughter, but the very comicality of these illustrations aided to fasten his main conclusion in men's memories.
In both these statements, imperfect as they were, great truths were embodied--truths which were sure to grow.
Lamarck's declaration, especially, that the development of organs is in ratio to their employment, and his indications of the reproduction in progeny of what is gained or lost in parents by the influence of circumstances, entered as a most effective force into the development of the evolution theory.
The next great successor in the apostolate of this idea of the universe was Geoffroy Saint-Hilaire.

As early as 1795 he had begun to form a theory that species are various modifications of the same type, and this theory he developed, testing it at various stages as Nature was more and more displayed to him.