What did Lamarck mean by lateral ramifications among species?

In the context of Jean-Baptiste Lamarck's evolutionary theory, he proposed that the animal kingdom was organized into a single, primary scale of increasing complexity, ranging from simple microscopic organisms to mammals.1

However, Lamarck recognized that species could not be arranged in a simple, linear series. He described species as forming "lateral ramifications" in relation to the general "masses" of organization represented by the various classes. These lateral ramifications occurred because species underwent transformations as they adapted to the diverse, particular environments to which they were exposed.1

Essentially, while the "power of life" drove organisms toward increasing complexity, the modifying influence of environmental circumstances caused species to deviate from that regular gradation, creating these lateral branches in the natural system of classification.1