Up to the time of the publication of Darwin's Origin of the Species (1859 ), most Christians believed that creation came literally as Genesis describes it, "Then God said, 'Let the earth bring forth all kinds of living creatures: cattle, creeping things, and wild animals of every kind.' And so it happened." (Gn 1:24) God created by his word and it happened.
Darwin began to wonder if the biblical account was true. He was very aware that breeders created all kinds of variations within species such as pigeons, dogs, cats, etc. and began to wonder whether on a longer time scale nature did bring about changes in plants and animals. After careful study and observations he proposed the theory of Natural Selection.
Darwin observed random mutations in shape, size, strength, armament, color, and behavior in individual animals of a specie. These alterations, he thought, were brought about by climate, food supply, sexual selection, use and non-use of parts, etc. He concluded that since beneficial changes were passed to descendents, nature helped these mutants to adapt to changing enviromments while inferior members of a specie would gradually die off. He called this Natural Selection. This has proven true and we can now appreciate the role of secondary causes at work in nature bringing about changes in living forms.
But his theory went further. He also maintained that small beneficial changes in a specie accumulated over a long period of time would lead to the emergence of different creatures. In this way, for example, birds could have evolved from reptiles by way of a common ancestor.
The general theory of evolution maintains the following: MATTER + ENERGY + RANDOM CHANCE = INCREDIBLY COMPLEX LIFE.
This vision places in nature itself all potentiality and intelligence to become. Nature has become the first and final cause of evolutionary change.
Due to new discoveries over the past 50 years in genetics, microbiology, information theory, etc., large evolutionary changes by natural selection have been called into question.