BUILDING BLOCK dimensional to 8-level
Newly hatched baby chicks peck at photographs of simulated grains, and strongly prefer them if lit as if from above. Turn the photograph over and they shun it. This seems to show that baby chicks ‘know’ that light in their world normally comes from above. But since they have only just hatched out of the egg, how do they know? Have they learned it during their three days of life? It is perfectly possible, but I tested it experimentally and found it not to be so. I raised chicks and tested them in a special cage in which only light they ever saw came from below. Experience of pecking grain in this upside-down world would, if anything, teach them to prefer upside-down photographs of solid grains. Instead, they behaved exactly like normal chicks raised in the real world with light coming from above. Apparently because of genetic programming, all the chicks prefer to peck at photographs of solid objects lit from above. The solidity illusion (and hence, if I am right, the ‘knowledge’ of the predominant direction of light in the real world) seems to be genetically programmed in chicks – what we used to call ‘innate’ – rather than learned as (I’m guessing) it probably is in us. (Dawkins, 2004)
One of the main differences between up and down in the world is the predominant direction of light. While not necessarily directly overhead, the sun’s rays generally come from above rather than below. This fact opens an important way in which we, and many other animals, can recognise solid three-dimensional objects. It works in reverse.
The photograph of moon craters is printed upside down. If your eye (well, to be more precise, your brain) works the same way as mine, you will see the craters as hills.
Turn the book upside down, so that the light appears to come from another direction, and the hills wil turn into craters that they truly are.
(About the primaeval worm, our ancestor of 590 million years ago.) Why is there a dorsal side and a ventral side? The argument is similar (as with the fore and aft asymmetry), and the one applies to starfish just as much as to worms. Gravity being what it is, there are lots of inevitable differences between up and down. Down is where the sea bottom is, down is where the friction is, up is where the sunlight comes from, up is the direction from which things fall on you. It is unlikely that dangers will threaten equally from below and above, and in any case those dangers are likely to be qualitatively different. So our primitive worm should have a specialised upper or ‘dorsal’ side and a specialised ‘ventral’ or lower side, rather than simply not caring which side faces the sea bottom and which side faces the sky. (Dawkins, 2004),