How did Lamarck’s theory describe a trend from simpler organisms to more complex organisms?

Jean-Baptiste Lamarck’s theory of organic development proposed that life naturally trends toward increasing complexity, a process he termed the "power of life" or the "cause that tends to make organization increasingly complex."1

Lamarck envisioned this as a single, regular scale of organization, starting from the simplest microscopic organisms and rising toward more complex life forms like mammals. He argued that the state of all animals is the product of two competing forces:1
* Increasing complexity: An inherent drive toward a more organized, regular gradation of life.1
* Environmental influence: The modifying effects of diverse circumstances, which disrupt this regular gradation by forcing organisms to adapt to specific environments.1

While Lamarck viewed the general progression as a scale, he noted that species do not form a simple linear series. Instead, they exhibit "lateral ramifications" as they undergo transformations to survive in particular environments. By adopting new habits in response to these environmental needs, organisms would use certain organs more or less, leading to physical changes that were then passed to their offspring. Although this theory is now scientifically discredited, it was a pioneering attempt to explain the diversity and development of life.12