I understand that the rules are not to quote a sutta at you, but the Buddha does have a nice agricultural simile for this one. 
He says that kamma is the field, consciousness is the seed, and craving is the moisture. So each deed (kamma) that you carry out during your life ends up as the soil for future lives. At the start of each new life, the seed of consciousness is planted somewhere in that field and fed by the moisture of craving (for sights, sounds, smells, tastes, touches, and ideas) it establishes itself into a new being. That seed of consciousness can be planted anywhere in the field of kamma. The place where it lands could be lovely soil rich in nutrients or it might be awful soil that is incapable of growing anything beautiful. Obviously the more good deeds you do over multiple lifetimes, the richer more lovely the field of deeds is overall, but you never know where the seed of consciousness is going to fall in the next life. It might just fall in a bit of old scrubland at the edge of the field that has been poisoned by old bad deeds.
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