Andrew - That's not neccessarily (spell?) true. As evolution doesn't work like that.
Take giraffes, for example. Giraffes used to ave short necks, but now they have long necks - evolution helped with that. Now, it doesn't mean that with each generation the giraffes neck gets taller.
Basically, whilst the embryo was developing the mothers womb it would have had a genetic mutation, this particular mutation would have most probably have been on a dominant allele, and the mutation would have giving the giraffe a longer neck.
At this time, there could have been a big growth in population on the number of giraffes, meaning that there would be more competition for food. However, as this mutated giraffe had a longer neck he/she will be able to reach the leaves on the higher trees. This meant that this giraffe was unlikely to die of starvation, and more likely to live and reproduce.
As the effected allele was probably a dominant one, this would mean that the giraffe will certainly pass on the taller neck to its offspring. And so on.
When scientists say that evolution is a gradual change, they don't mean gradual as in with each generation the giraffes neck gets taller. It means that in each generation a higher percentage will have the taller neck.