Darwin's credible and incredible critics; New biology vs. fundamentalism; Abusing Science: The Case Against Creationism, by Philip Kitcher. Cambridge, Mass.: The MIT Press. 213 pp. $15.; Darwinism Defended: A Guide to the Evolution Controversies, by Michael Ruse. Reading, Mass.: Addison-Wesley. 356 pp. $12.50 (paperback).

Evolution as theory, fact, and process is the great unifying theme of modern biology. The theory itself may evolve and change. New facts may come to light. And scientists may argue heatedly over the nature of its processes. But in its grand sweep and design, in its ability to relate details to the tapestry of earthly life as a whole - in short its ability to give meaningful perspective to an entire branch of science - evolution has done for biology what Einstein and his successors have tried to do for physics in their search for a unifying theory of basic forces.

Why, then, is there so much controversy over it?

Among scientists, the vigorous arguments over the way evolution happens and over interpretation of the data is a sign of healthy research. There is little doubt that earthly life has evolved from simpler forms over the past several billion years.

Fundamentalist Christian objections are another matter. This is an effort, both ominous and comic, to enthrone the literal interpretation of biblical creationism as a science and to force its teaching in the public schools. Here the controversy is over the separation of church and state.

These two books provide a solid background for the nonscientist in the issues of both controversies. While admittedly written from definite points of view, both of them are thorough and authoritative. However, they do have somewhat different emphases.

In ''Darwinism Defended,'' Prof. Michael Ruse (University of Guelph) is more concerned with evolution as science than as an object of political debate, although he does not neglect this aspect. He is a committed Darwinian, incisively defending Darwin's approach to evolution with wit and charm. However, his overview of the theory and its development since Darwin and of differing scientific views is a well-balanced nontechnical briefing.

He boldly takes on scientific dissents from pure Darwinism - claims that evolution proceeds by great leaps rather than gradual change, that random genetic drift rather than natural selection is the decisive evolutionary mechanism, that evolutionary theory is more of a metaphysical research program than true science, because it is hard to put it to the test.

In all aspects, according to Ruse, the foundation Darwin laid and the theory that has been erected on it are one of the great scientific achievements of all time. He is right about this latter point. But readers should remember that scientists who hold other views on the mechanisms of evolution and the nature of the theory are not ready to admit defeat.

The bulk of the book (the first 281 pages) are devoted to this exposition. It is in the final chapters that Ruse grapples with so-called ''scientific'' creationism. Taking up its claims point by point, he explains why he believes them to be ''totally, utterly, and absolutely wrong.''

For Philip Kitcher, on the other hand, this is the main theme of his book, ''Abusing Science.'' He, too, gives a readable overview of modern evolutionary theory, of the facts that support it, and of the various points of scientific controversy. However, his main effort is to examine, and refute, the creationist arguments in detail.

Thus, for example, he thoroughly explains why scientists have confidence in their methods of dating the age of fossils and of Earth - methods creationists claim to be unreliable. Also, he argues at length that efforts to have creationism accepted as a science would not only degrade our concept of science by introducing irrationalism but would undermine sound education. This is in addition to violating the separation of church and state, were schools to teach creationism as science.

Both of these books are easy to read and thought-provoking. Readers who would like to compare the creationists' own statements may want to follow up some of the extensive references the authors give to creationist literature. I doubt, however, that this would do more than reinforce the authors' conclusions.

Ruse is right when he says, ''Scientific creationism is fallacious by every canon of good argument. Thus I say keep it out of the schools.'' And, as for fears that evolution study undermines human dignity, is not Kitcher right when he notes that, from a human point of view, ''The theory of evolution explains to us what our ancestry has been. It does not explain away our worth. Why should we be afraid to learn more about what we are?''