How were embryos important to Darwin's research?

The embryos help Darwin find that finches came from the same ancestor which they go through phases of evolutions, this depends on the island the finches are located. Darwin discover that in different islands the beak from the finches were bigger hence stronger to crack difficult shells from plans, but in a different island finches have smaller beaks to get nectar from that plants. Darwin came to the conclusion that finches adapt from their environment.