Why did Lamarck believe that the simplest life forms arose through spontaneous generation?

Jean-Baptiste Lamarck believed that the simplest life forms arose through spontaneous generation because his theory of organic transformation required a continuous source of new, simple organisms to populate the bottom of his proposed scale of nature.1

Lamarck envisioned a "natural" system of classification organized as a single, linear scale of increasing complexity, ranging from the simplest microscopic organisms to mammals. In his view, life was characterized by a "power of life"—an inherent tendency for organization to become increasingly complex over time. Because he posited that all life was constantly evolving toward higher levels of complexity, he needed a mechanism to explain the ongoing existence of the simplest forms. He concluded that these basic organisms were being continually produced from nonliving matter, thereby replenishing the start of the evolutionary scale as older, simpler forms evolved into more complex ones.12