To Lamarck, says Haeckel, "will always belong the immortal glory of having for the first time worked out the Theory of Descent as an independent scientific theory of the first order, and as the philosophical foundation of the whole science of Biology." He taught that in the beginnings of life only the very simplest and lowest animals and plants came into existence; those of more complex structure developing from these; man himself being descended from ape-like mammals. For the Aristotelian mechanical figure of life as a ladder, with its detached steps, he substituted the more appropriate figure of a tree, as an inter-related organism. He argued that the course of the earth's development, and also of all life upon it, was continuous, and not interrupted by violent revolutions. In this he followed Buffon and Hutton. Buffon, in his Theory of the Earth, argues that "in order to understand what had taken place in the past, or what will happen in the future, we have but to observe what is going on in the present." This is the keynote of modern geology. "Life," adds Lamarck, "is a purely physical phenomenon. All its phenomena depend on mechanical, physical, and chemical causes which are inherent in the nature of matter itself." He believed in a form of spontaneous generation. Rejecting Buffon's theory of the direct action of the surroundings as agents of change in living things, he sums up the causes of organic evolution in the following propositions: 1. Life tends by its inherent forces to increase the volume of each living body and of all its parts up to a limit determined by its own needs. 2. New wants in animals give rise to new movements which produce organs. 3. The development of these organs is in proportion to their employment. 4. New developments are transmitted to offspring.