Organic Evolution

The word evolution comes from the Latin e (from) and volvere (to roll), literally meaning the act of unrolling or unfolding — an apt image for a process that brings about slow, orderly changes carrying one form gradually into another, so that descendants end up different from their ancestors. Organic evolution, more precisely, can be defined as the slow, gradual, continuous and irreversible set of changes through which today's complex forms of life developed from their simpler, pre-existing ancestors. Charles Darwin captured this idea in a famous phrase: evolution is 'descent with modification'.

Before Darwin's theory, an earlier explanation had already been proposed and studied: Lamarck's theory of the inheritance of acquired characters (already covered in Std 10). According to Lamarck, an organism's traits are acquired in response to an internal driving force, to changes in its environment, to new needs that arise, and to the use or disuse of particular organs — and, after being acquired within a single lifetime, these traits are then passed on to the next generation, so that after many generations a new species eventually results. …