Definitions of Evolution
• Charles Darwin defined evolution as ‘’descent with modification (1859: “On the Origin of Species”)
• Herbert Spencer “Evolution is a change from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity through continuous differentiation and integration’’ (1893: “Principles of Sociology”).
• Julian Huxley defines evolution in an extended sense: “as a directional and essentially irreversible process occurring in time which in its course gives rise to an increase of variety and an increasingly high level of origination in its products”
• G.H. Hardy and W. Weinberg depict evolution as “changes in the gene pools of populations”.
• Buettner-Janusch defines it as “changes in gene frequencies between ancestral and descendant populations” or “changes in the morphology of organisms through time” or “changes in number and kinds of animals within major lineages”
Pre Darwin Theories
Theory of spontaneous generation or Abiogenesis
The theory of spontaneous generation is as old as human thought. It is well known that life arises only from pre-existing life (principles of bio-genesis) and assumes that life originated from inert, inorganic matter as a result of a series of physico-chemical conditions which must have existed at a given moment during the evolution of earth. According to this theory worms generated from manure; insects from dew, rotten slime, dry wood, sweat and meat; frogs and salamanders from coagulated slime; and toad, snakes and mice from the mud of the river Nile. The scientists Aristotle, Thales, Plato and Von Helmont believed this idea of abiogenesis until the 17th century. Later this theory was disproved by Francess Redi, Spallanzani and Louis Pasteur.
Theory of Eternity of Present Condition
This theory argues for the unchangeableness of the universe. It holds that organisms remain unalterable throughout their individual existence and will continue in the same unchanging state throughout eternity.
Theory of Special Creation or Creationism
According to this theory the living organisms on the earth were created by divine power in six days. Father Suarez (1548-1671) is strong supporter of this theory. The created organisms exist unchanged from the day of their creation. This concept of special creation was followed until middle of the 19th century.
Theory of Catastrophism
Cuvier (1769-1832) and Osbinge advocated that the earth was subjected to periodic catastrophes. These catastrophes destroyed the life from time to time and created new and special form of life after each destruction.