Lamarck's theory of Progressive Evolution

Lamarck hypothesized that simple organisms arise repeatedly (yellow circles), and that more complex organisms (red circles) arose earlier than simpler. Evolution produces a hierarchy from simple to complex, as seen in the progressive, time-dependent series (yellow to red) in the right-hand column. This theory accounts for the observed Scala Naturae (Chain of Being), without suggesting that observed simpler organisms (e.g., invertebrate animals) gave rise directly to more complex (e.g., vertebrate animals).