Butterflies undergo complete metamorphosis, and their life cycle contains 4 stages.
First, they are born as an egg.
Then, they are hatched as a larva. They now eat flowers and leaves and will molt constantly.
Next, they become a chrysalis, or a pupa. This is a resting stage.
Then is the long awaited adult stage, where butterflies take the form of the beloved, beautiful, winged insects we've come to admire. They no long grow and will now reproduce and repeat the cycle.
The most consistent way to obtain information about the evolution of species is to look at the fossil record. Another is to observe the differences between species currently on earth. Another is to use carbon dating.