This was the first time I have seen Darwin's blog and it ate up much of my afternoon.

I'm glad to hear that; that was one of the goals - to introduce LW to Darwin a bit.

Does Darwin ever post on LessWrong, and if not I would be curious why not?

LW is a very recent thing. Darwin got involved in cryonics in, like, the 1960s. It's not surprising if, as he began polishing and dumping online what sometimes feels like decades of material, he didn't do so on some popular new transhumanist website; so there may be nothing there to explain. If there was, it may be that Darwin differs philosophically from LW in general (certainly Yudkowsky has vociferously criticized the excerpted post).