It was discredited by Darwin in 1859.
Back then, it was known from the fossil record that some kind of evolution (so-called "Law of faunal succession") had occured, it was just that no-one had any idea how it worked. Orthogenetic theory and Lamarckism proposed ad hoc explanations without any mechanism.
Then Darwin made the simple observation that the same mechanism that allows pigeon breeders to breed new breeds of pigeon is also at work in the natural world. He hypothesised that this accounts for all the diversity of life on earth.
He elevated this hypothesis to a theory by supporting it with lots of evidence, and by giving examples of how to falsify it.
Perhaps someone could post a link to a nice summary... Here's one
Not that I care that much about the orthogenetic principle in particular, but it's my understanding that orthogenesis continued to have its adherents until well into the 1950s.
And in fact Darwin's ideas about the mechanism of natural selection did not by themselves negate the notion that there was some sort of guiding force (not divine) that was part of the natural selection mechanism.
And if you count the anthropic principle, then it is still alive. Actually, I just found some very serious people still proposing modern versions of orthogenesis (http://www.complexsystems.org/essays/ReviewComplexity.htm)... don't know anything about them so if you find out that their great grandson's nephew's brother-in-law once quoted some evolutionist out of context, try to contain yourself.
A very interesting related work is by Robert Wesson, called Beyond Natural Selection, where he provides a fairly devastating critique of natural selection as a mechanism to explain the evolution of life all on its own, and proposes some ideas derived from modern theories of complex systems and self-organization that are not completely dissimilar from these ideas.
He elevated this hypothesis to a theory by supporting it with lots of evidence, and by giving examples of how to falsify it.
Ya, but you know, he wasn't trained as a biologist, but rather as a religionist, so I don't think he was qualified to speak to issues of biology.
Perhaps someone could post a link to a nice summary... Here's one
Hmmm... your link deals almost entirely with pre-Darwin, so hardly provides a catalog of those aspects of evolutionary theory which have been discredited since Darwin. You really want to claim that the theory of evolution has not undergone any modifications since Darwin? None of it has had to be revised to take into account the evidence discovered since then?
I didn't know you were bringing in Debussy and / or Narnia into the discussion ;-)
Cheers!
...and Merry Christmas!