Frings and Frings¹ list six postulates of the Darwinian view of evolution.

First, Darwin assumed that there was a tendency for animals and plants to reproduce geometrically. By this he meant that a pair of animals had far more offspring than was necessary to replace themselves. A fish may lay millions of eggs in its lifetime. If each egg produced a full-grown adult, the population of that species would swell ominously in a short time.

Darwin's second postulate was that the number of individuals in a species remained relatively constant.

The third postulate was that since large numbers of the offspring failed to reach maturity, there was a struggle or competition for food and reproduction.

Darwin further assumed that there was variability between individuals and that the variability was unlimited.

The fifth assumption was that natural selection allowed only the "fittest" to survive.

Finally, Darwin assumed that the environment changed continuously so that the definition of what is "fittest" changed with time.

Darwin also believed that the process of change was gradual.
He stated,

"If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous successive, slight modifications, my theory would absolutely break down."²

Other writers, since, have departed from the gradual view of evolution and have embraced a more rapid cataclysmic view of how species evolve.
In the next few chapters we will examine these postulates and the speed with which evolution occurs.

NOTES

1. Frings and Frings, Concepts of Zoology, New York: McMillan, 1970, p. 53, 54.
2. Charles Darwin, The Origin of Species, New York: New American Library, 1958, p. 171.