How did Lamarck arrange animals in his natural system of classification?

Jean-Baptiste Lamarck’s natural system of classification was primarily organized as a single, linear scale of increasing complexity. This scale encompassed all classes of animals, beginning with the simplest microscopic organisms, which he termed "infusorians," and ascending to the most complex, the mammals.1

Lamarck viewed this arrangement as a product of the "power of life," a force that naturally drives biological organization toward greater complexity. However, he noted that this regular gradation was often disrupted by the influence of diverse environmental circumstances, which caused species to deviate from the main scale. Consequently, while the classes followed a hierarchical progression, Lamarck described species as forming "lateral ramifications" relative to these general masses of organization, reflecting the specific adaptations they acquired in response to their unique environments.1

In addition to this broad theoretical framework, Lamarck made significant practical contributions to taxonomy, such as introducing the distinction between vertebrates and invertebrates and separating spiders and crustaceans from the class of insects.2