According to Lamarck, two principal forces produce evolutionary change. In Histoire Naturelle des Animaux Sans Vertebres (1815) he explained that
Everything rests here on two essential foundations which regulate the observed facts… (i) On the power of life, whose results are the increasing constitution of the organisation, and as a consequence, the mentioned progress; (ii) On the modifying cause, whose products are interruptions, diverse and irregular deviations in the results of the power of life. 
The phrase "power of life" has been taken by many to mean some form of supernatural or metaphysical interference. Lamarck was hampered by his ignorance of the processes that cause changes in organisms, a situation which still holds today in spite of our much superior knowledge of genetics and biochemistry. Lamarck stated four laws concerning processes in evolution, two in Philosophie Zoologique and two in Histoire Naturelle des Animaux sans Vertebres First, "Life through it own powers, tends continuously to increase the volume of each body which posses it, and to extend the dimensions of its parts, until an end point which she determines herself." This "law" explains nothing — it merely states what has been observed. Second, "The formation of a new organ in an animal body results from new incidental need which is incessantly felt, and a new movement created and supported by this need". Lamarck appears to be saying that if an animal moves into a new environment, or the environment changes, the animal needs to adapt in order to survive. This law, which has poor predictive power, is almost impossible to demonstrate, but it was necessary if his third law was to be valid — namely, that "the development of organs and their power of action are always related to the use of the organs." If the second and third laws are combined into the phrase "law of use and disuse," then it is no longer an evolutionary process but an adaptation caused by adjustment of the organism to changed circumstances. Lamarck considered that his laws were supported by observations of organs in some animals that had either become enlarged as in the neck and legs of the giraffe, or had disappeared altogether as in blind fish in caves. In reality, if his second and third laws operated as he claimed they did then evolutionary processes would operate very quickly, but Lamarck denied this: "I know very well that for us the appearance must present…..a stability which we believe to be constant, although it is not really so; for a rather large number of centuries may be a lapse of time too short to make the mutations about which I speak so extensive that we can observe them" (Discours douverture, 81).
Lamarck had got himself into a bind because he thought that evolution only occurred by incremental steps. He therefore tried to salvage his position with his fourth law: "Everything which has been acquired, outlined or changed in the organisation of the individuals in the course of their life, is preserved through their reproduction, and is transmitted to the new individuals which sprang from those who have undergone these changes". This is his famous law of the inheritance of acquired characteristics for which he is most remembered. It has long been a controversial idea that was and is denied by many biologists, but there is a growing body of evidence in the form of epigenetics which indicates that there may be some truth in it.
In Lamarck's theory of evolution, subtle invisible "fluids" composed of heat or electricity which had become "modified and animalised" on entering the bodies of higher organisms, created hollows or cells. The fluids circulated by channels or canals and in doing so created vessels and cavities. They also deposited certain substances which accumulated in special places to form organs. It followed that if certain organs or limbs were used more frequently in life then these would be subject to greater movement of fluids through them and they would become enlarged. These larger limbs or organs could be passed on to their offspring. Equally if organs or limbs were not used the flow of fluids would nearly cease and the parts of the body affected would contract and even disappear.
It is important to remember two points with respect to this idea. First, that when that Lamarck wrote, heat and electricity were not understood as they are today. By substituting the words energy and nutrients for fluid we may get a clearer understanding of what Lamarck was trying to explain. Second, the inheritance of these enlarged organs was a commonly held belief at the he wrote, and it was neither original or unique to him. Lamarck was simply trying to offer an explanation of a then-common belief.
Last modified 24 January 2017