Did Holy Prophet صلى الله عليه وسلم encourage or discourage marriages between cousins over several generations?

What does Islam say about marrying your 1st cousins for several generations? Is it encouraged or discouraged? **Edit:** There is a practice in some Asian countries that families tend to marry within themselves for many generations such that upto 3-4 generations have couples who were first cousins. Science today tells us that marriages within the family over many generations contracts the gene pool and may lead to diseases and deformities. **I'm aware that Islam allows marriages between cousins. But does it allow encourage or discourage their children, grand children, great grand children and so on to again marry just their cousins?**