In Darwin's 'The Origin of Species', he did say
“If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection."
However, scientific advancements since the publication of the book has build upon the concept of biological evolution. It has discredit some of Darwin's statements like the quote above and provided further insight in others.
Remember that not everything that Darwin says is absolute, it is a common case for any 'father of -something-'.
Example: Democritus - he was widely credited for the idea that all matters are made up of atoms and cannot be subdivided (contributed to the theory of atoms). We now know that is false since the atom is made up of protons, neutrons and electrons.
As for explaining the basic of co-evolution:
University of California provides an example using insects and plants with explaining the basic of how co-evolution can occur between the two.
http://evolution.ber...evolution.shtml
Its a short explanation (1 page).