Interbreeding is one of the methods of natural evolution, but without man's interference, whether or not a strain could survive depended on its ability to adapt, on whether it could pass the test of survival of the fittest.
不同品系的同種動物交配，原也是自然演化的方式之一，但在人類介入以前，一個品種能不能留存下來，取決於它適應環境的能力。