Darwin's theory essentially holds that every species has variations. Some varieties turn out to be advantageous for competition in survival, breeding more, etc., while other varieties are not. Creatures with advantageous traits live longer, reproduce more and pass on the useful traits. This natural selection caused by environmental pressures causes species to develop in particular directions and, in time, to lead to the development of radically different species. All life is descended from a few primordial forms in a gradually branching and diversifying 'tree of life'.
Some of this is true, some of this is not. This is why evolution is deceptively believable at first glance until one looks into it deeper. Yes, every species has variation within its bounds. This is true, and it can be readily confirmed by the diversity of human beings on this planet, and the diversity of different types of dog breeds, ranging from a Great Dane to a Chihuahua. Darwin believed that changes could be extended without limit. What evolutionists do is rely on an extrapolation that varieties within the species can, over huge periods of time, become separate species in their own right. The question is whether this is true. Many qualified people have denied this:
Luther Burbank (American botanist and horticulturalist responsible for developing more than 800 strains and varieties of plants):
>I know from experience that I can develop a plum half an inch long or two-and-a-half inches long, with every possible length in between, but I am willing to admit that it is hopeless to try to get a plum the size of a small pea, or one as big as a grapefruit, I have roses that bloom pretty steadily for six months of the year, but I have none that will bloom twelve, and I will not have. In short, there are limits to the development possible.
(from Norman Macbeth's Darwin Retried)
Pierre Grassé (French zoologist and author of over 300 publications):
>In spite of the intense pressure generated by artificial selection (eliminating any parent not answering the criteria of choice) over whole millennia, no new species are born. A comparative study of sera, hemoglobins, blood proteins, interfertility, etc., proves that the strains remain within the same specific definition. This is not a matter of opinion or subjective classification, but a measurable reality. The fact is that selection gives tangible form to and gathers together all the varieties a genome is capable of producing, but does not constitute an innovative evolutionary process.
(quoted from Philip Johnson's Darwin on Trial)
Francis Hitching (British author):
>It is now absolutely clear that there are firm natural limits to what can be done. Remarkable achievements can be made by crossbreeding and selection inside the species barrier, or within a larger circle of closely related species, such as wheats. But wheat is still wheat, and not, for instance, grapefruit. Between 1800 and 1878, the sugar content of beets was raised from 6 to 17 per cent. A half century of further breeding failed to make any difference.”
(quoted from Francis Hitching's The Neck of the Giraffe)
Things like bacteria and fruit flies are also ideal for this sort of study in trying to extrapolate varieties into entirely new kinds of organisms, since they reproduce so quickly. Despite cycling through thousands and thousands of generations of these organisms, there has never been any evidence of speciation among them from normal bacteria or normal fruit flies. Sometimes they come out and announce that they have done so, but invariably it is a variety that they selected for and nothing else (the same thing dog breeders do). One would think in a scientific, highly-controlled setting like this, that they would be able to make radical changes quickly. Evidence doesn't support this.
It's also important to point out that instead of natural selection being seen as an innovative force as Darwin had postulated, some biologists, such as proponent of design Edward Blyth (who comes before Darwin, notably), wrote of natural selection as a conservative force, not an innovative one. Natural selection maintained the stability of the species within its environment, it functioned as a sort of quality control, removing defective varieties, while healthier and more well-adapted creatures would survive and reproduce. Essentially, Natural selection helps a species flourish by favoring gene combinations that allow it to adapt to new and changing conditions.