In his introductory lecture, given in 1803, after ten years of work on the lower animals, he addressed his class in these words: "Indeed it is among those animals which are the most multiplied and numerous in nature, and the most ready to regenerate themselves, that we should seek the most instructive facts bearing on the course of nature, and on the means she has employed in the creation of her innumerable productions.

In this case we perceive that, relatively to the animal kingdom, we should chiefly devote our attention to the invertebrate animals, because their enormous multiplicity in nature, the singular diversity of their systems of organization and of their means of multiplication, their increasing simplification, and the extreme fugacity of those which compose the lowest orders of these animals, show us, much better than the higher animals, the true course of nature, and the means which she has used and which she still unceasingly employs to give existence to all the living bodies of which we have knowledge." During this decade (1793-1803) and the one succeeding, Lamarck's mind grew and expanded.

Before 1801, however much he may have brooded over the matter, we have no utterances in print on the transformation theory.
His studies on the lower animals, and his general knowledge of the vertebrates derived from the work of his contemporaries and his observations in the Museum and menagerie, gave him a broad grasp of the entire animal kingdom, such as no one before him had.

As the result, his comprehensive mind, with its powers of rapid generalization, enabled him to appreciate the series from monad (his _ebauche_) to man, the range of forms from the simple to the complex.

Even though not a comparative anatomist like Cuvier, he made use of the latter's discoveries, and could understand and appreciate the gradually increasing complexity of forms; and, unlike Cuvier, realize that they were blood relations, and not separate, piece-meal creations.