Postulates.

In this book Darwin developed two main points:

Descent with Modification:

Unity of life.

Darwin believed in perceived unity in life, with all organisms related though descent from some common ancestor that lived in the remote past.

Tree like History.

In the Darwinian view, the history of life is like a tree, With multiple branching and rebranching from a common trunk all the way to the tips of the living twigs, symbolic of the current diversity of organisms. At each fork of the evolutionary tree is an ancestor common to all lines of evolution branching from that fork.

Natural Selection and Adaptation:

Darwin suggested that populations of individual species become better adapted to their local environments through natural selection. Darwin‟s theory of natural selection was based on the following observations. Production of more individuals than the environment can support, leads to a struggle for existence among individuals of a population, with only a fractions of offspring surviving each generation. Survival in the struggle for existence is not random, but depends in part on the hereditary constitutions of the surviving individuals. Those individuals whose inherited characteristics fit them best to their environment are likely to leave more offspring than less fit individuals. Selection of best. This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favourable characteristics accumulation over the generations thus leading to the evolution of a new species.