the other thing darwin does well is spell out the inter-relatedness of species.
for instance how the number of cats in an area effects the number of flowers in that same area.
more cats = less mice = more bees (apparently mice go after honey) = more pollination.
now maybe this has been stated before, but hes just showing the infinate amount of variables that can attribute to circumstances that can account for the flora in an area, and therefore the fauna that follow it.
maybe this is all rhedementary, but seeing it spelled out is somewhat more stimulating than hearing it from a stupid prof.