Its methods are observation and experiment.

It observes the tiny cell on the surface of an egg yolk, and watches it divide and multiply until it becomes a great mass of cells, which group off or differentiate, and rearrange and alter their shapes.
It observes how little organs unfold themselves, or evolve out of these little cell groups--how gradual, but how unvarying the change; how one group becomes a bone, another a brain, another a muscle, to constitute in three short weeks the body of a matured chick.

Those little tendons like silken threads, that run down those slender pink legs to each and every toe, and move its little joints so swiftly that we hardly see them--that little brain, no bigger than a tiny seed, in which is planted a mysterious force that impels it to set all those brand-new muscles in motion, and to dart after a fly with the swiftness of an arrow--all this wondrous mechanism, all this beauteous structure, all this perfection of function, all this adaptation to environment, have evolved from a few microscopic cells in three short weeks.
Biology is the science that observes all this, and enunciates the law that the life history of this animal cell, _i.e._, its history from a simple unicellular state in the egg, to its complex multicellular state in the matured chick, represents the history of the race to which the chick belongs.

If we could trace that chicken back through all its ancestry, we would discover at different periods in the history of life upon the globe (about 100 million years, according to Haeckel) exactly the stages of development we found in the life history of the chick, and arrive at last at a primordial cell.
What is true of the chick is true of all life.

This is the law of evolution.