Envy

That's the thing, isn't it. Take species' from anywhere in the Universe and any when in Time, and they'll be completely different. Some might have tentacle, or wings, or no external limbs of any kind. They could have bright pink nostrils, revolving eyeballs, or be able to play symphonies on their teeth. Their intelligence would range from that of a unicellular blob of microscopic goo clinging to a rock on some volcanic Hades of a planet to the mind-blowing mathematical calculations of the Brains of Ek. But it's always the same. If someone else has got something they don't, they want it.

He'd read a study once, some students on Old Earth searching for a 'comprehensive understanding of the causes and effects of envy in society, and its place in human development throughout history'. A bloody good piece of work actually, but it still left him unsure about things. They had decided that the amount of envy felt by a species, and the effect it had, was directly proportional to that species' intelligence.

The reason was that a more intelligent group of creatures would see something that their neighbours had, then use those fantastic brains of theirs to think of all the things they could do with it if they had it. That would make them want the something even more, and by using those fantastic brains they'd think of how best to get it for themselves. Humans being the 'most intelligent' creatures on Earth at the time, the students gave their own species as an example of what envy can cause them to do. Twenty thousand years of apes crawling across the surface of Earth has produced a lot of things for them to be jealous about: food, or land, or oil... It's also produced some very intelligent and well-thought-out ideas about getting it. Politics, and guns, and fire, fire, fire...

This was true, of course.

But backtrack to that unicellular blob of microscopic goo clinging to a rock. Ours might not be the only one on that rock; it might have a neighbour. So over the weeks and months our blob of goo and its neighbour live on the rock, quietly ignoring each other's existence. Neither's done anything to the other, so what's the point in getting anxious?

Then one day, the sulphurous wind comes howling down from the nearby volcanic plateau. It races over our rock, and just happens to deposit a little bundle of life in a crack in the stone. The tiny seeds sprout and grow, anchoring itself to the rock. They die and create a bit of soil, where more and more seeds can grow. Eventually we have ourselves a moss or lichen. When it reaches it, our little blob of goo is free to gorge. With this feast inside it, it grows and reproduces into a slightly larger blob of goo. But it's been too greedy - the lichen is dying and shrinking. But the plant begins to grow around its neighbour. Once having tasted, our blob needs more of that sweet, sweet juice. So slowly and surely it squelches along towards its neighbour sitting in luxury.

Then devours it.

You didn't need intelligence for envy - it was part of being alive. How you got what you wanted just depended on how big you were, or how fast you were, or how many nasty horns you had sticking out of you. A few extra brain cells were useful for the ones who didn't have anything like that. With a little more brain power they could sharpen up a rock, or plan an ambush, or even form a political party. In that case it just depended on how many people were willing to die instead of you. The mob often created it's own reasons - they all had their own green-eyed monster, nipping at the heels of the flock. Envy controlled things.

Yet the Doctor had never thought it could control him. Definitely not this barbaric, selfish, cruel form of it. Not this ancient, stupid, human feeling. 'Let copulation thrive' - a vision that terrified him. This envy...it caused him to lose his reason, his restraint, who he was. It made him a bully and an idiot, who got waist deep in shit and just kept digging. He hated that.

But surely it was worth it?

It kept them away from her, all those pretty-boys she was so prone to attracting. All devilish smiles and shiny hair. It kept her safe, not trusting the guys. It kept her with him.

He knew that some day, something inside him would snap. It had been a close thing when he'd first met Jack. The Doctor's sarcasm, cynicism, and blatant disrespect had been all that was between Jack and a punch in the face.

But some day someone would go too far, and things would be done and said that he could never back out on. Even if she never mentioned it to him again, those feelings and actions would still be between them. If those remained there, never mentioned and never rescued... The Doctor knew that whenever he died, he would die with a broken heart.

-Sir, let me kiss your hand

-Let me wipe it first, it smells of mortality.