Tony, thanks so much for taking time to respond. I know that you have knowledge on this topic, so correct me at any time.
Logically is to assume that finches originated from S. A. Mainland, and how they come to island is no importance at this time. By adaptation they separated into different sub-specie.
Now, because there were no element “hybridization” with other sub-finches (birds with the same genus), evolution (known to the Mainland) in one way stopped there. Mutation at the specific gene-area helped development of the sub-specie (different beak).
Regards the book, why should I buy it when I can borrow from you (indefinitely