What is "to breed in and in"

Breed \Breed\, v. i.

1. To bear and nourish young; to reproduce or multiply itself; to be pregnant.

   That they breed abundantly in the earth.
   --Gen. viii. 17.

   The mother had never bred before.
   --Carpenter.

   Ant. Is your gold and silver ewes and rams? Shy. I can not tell. I make it breed as fast.
   --Shak.

2. To be formed in the parent or dam; to be generated, or to grow, as young before birth.

3. To have birth; to be produced or multiplied.

   Heavens rain grace On that which breeds between them.
   --Shak.

4. To raise a breed; to get progeny.

   The kind of animal which you wish to breed from.
   --Gardner.

   To breed in and in, to breed from animals of the same stock that are closely related.