It never could be inferred from the facts stated, even did nature correspond to Lamarck's description.
But nature does not correspond to Lamarck's description.

That description corresponded moderately, perhaps, to the science of his day, which was based chiefly upon external resemblances; but no scientific naturalist of the present day would accept it as a correct statement of the facts revealed by modern science.
In the first place there is no such imperceptible blending and shading off of species as the description would imply, obliterating all distinctions of species, and rendering it impossible even for a naturalist to distinguish one species from another.

Since the time of Lamarck, structure and physiology have been more studied than mere external appearances; so that from a tooth or bone Cuvier or Agassiz could reconstruct an animal, and indicate its internal organization, as well as its form and habits.

But even in Lamarck's days, and even to the most uneducated, there was no such imperceptible shading and blending as the theory requires.

It is well to look here at its requirements, for they are not fully presented by its friends.