In 1801 appeared his _Systeme des Animaux sans Vertebres_, preceded by the opening discourse of his lectures on the lower animals, in which his views on the origin of species were first propounded.

During the years 1793-1798, or for a period of six years, he published nothing on zooelogy, and during this time only one paper appeared, in 1798, on the influence of the moon on the earth's atmosphere.

But as his memoirs on fire and on sound were published in 1798, it is evident that his leisure hours during this period, when not engaged in museum work and the preparation of his lectures, were devoted to meditations on physical and meteorological subjects, and most probably it was towards the end of this period that he brooded over and conceived his views on organic evolution.
It appears that he was led, in the first place, to conchological studies through his warm friendship for a fellow naturalist, and this is one of many proofs of his affectionate, generous nature.

The touching story is told by Etienne Geoffroy St.Hilaire.[36] "It was impossible to assign him a professorship of botany.

M.de Lamarck, then forty-nine years old, accepted this change in his scientific studies to take charge of that which everybody had neglected; because it was, indeed, a heavy load, this branch of natural history, where, with so varied relations, everything was to be created.