One issue always challenges my vegetarianims: animals are killing and eating the hell out of each other. Tonight watching "Animal Planet" on my parents' extensive cable package, I saw hyenas, hippos, crocodiles, and lions eating the raw flesh of recently killed animals. If, in nature, animals are killing and eating each other, is there really legitimacy to a stance that it is always wrong for humans to eat the flesh of animals?
Now, "Nature, red in tooth and claw" will not lead me to again abandon vegetarianism. I don't think nature has much at all to do with genetically altering a chicken, imprisoning it in a tiny cage for its entire life, mutilating it, torturing it, and finally killing it so that I can eat cheap wings. The way humans consume animals most certainly does not have much to do with nature.
Expanding on the line from Tennyson's poem, how can evolution help us know about humans eating meat? In fact, it doesn't. We do know that biologically there is not a necessity for humans to eat meat (while there is for some animals, such as cats). But evolution creates two contradictory moral arguments about animal consumption:
1. Since evolution tells us how close we are to the animals, we should really show more respect and concern for them.
2. Since evolution tells us that humans have proven to be the strongest species, we have every right to use animals that are inferior to us however we choose (and furthermore, evolution suggests there isn't any morality to worry about, anyway).
So I don't think we can look to evolution for answers to the question of vegetarianism. But of course, the question of "how should we treat animals ethically?" doesn't end with "should we or should we not kill and eat animals?" but extends further to how those animals that are raised, killed, and eaten are treated.