I got into a little usage discussion with a commenter on Tracey Taylor's fine blog about East Bay real estate. (Tracey was riffing on a post I wrote about the meaning of Main Street, so yeah, this is what you might call logrolling.)
I knew I was right, but I wasn't sure why. So I did some research.
Commenter David used the word homogenous in his assertion that Berkeley and San Francisco lacked true demographic diversity. I responded that, regardless of whether the assertion is true, the correct word here is homogeneous. David apparently thought I was critiquing his spelling; he came back with a dictionary definition for homogenous and a boast that he was the Wisconsin State spelling bee runner-up.
Yes, homogenous is a dictionary word. David spelled it correctly. It just isn't the right word in this context.
Homogenous (emphasis on the second syllable) means "similar in makeup because similar in descent." The Columbia Guide to Standard American English (1993) gives this example: "These animals are homogenous, as their similar physiology makes clear." Words @Random, from dictionary publisher Random House, explains that homogenous is a technical term in biology ("The forelimbs of mammals and fishes are homogenous").
Homogeneous (emphasis on the third syllable, with a long e vowel) means "of the same kind or structure; of like composition." CGSAE's example: "It was a homogeneous community, its members holding remarkably similar values." (Or: homogenized milk, in which the fat globules are thoroughly emulsified throughout the liquid. Note: In my original post, I had this example in the wrong place. See Regan's comment, below.) That's what David meant, and why his word choice should have been homogeneous.
Both homogenous and homogeneous are composed of Greek word parts that mean "of the same kind." But, notes Words @ Random, "Homogenous is properly limited only to this biological use, so if you're not writing about this, the word you want is homogeneous."
The opposite of homogeneous is heterogeneous. The opposite of homogenous, rarely seen outside scientific literature, is heterogenous.
Hope I've cleared that up.