The Last Word
The Divinely imposed mixture of wild beasts could have led the Jewish people to believe that God is in favor of mixtures. Perhaps this was a sign that all barriers can be broken down, and that Jew and non-Jew can mix alike?
Therefore, at the same time God sent a mixture, He also "set apart the land of Goshen where My people remain, so that there will not be any harmful beast there" (v. 18).
This teaches us that even when a Jew goes out into the wider world of "mixed values" and blurred boundaries, he should not come to the conclusion that God favors this mixing. Rather, it is the Divine will that the Jewish people and their values should remain distinct("set apart") from the rest of mankind.
(Based on Likutei Sichos vol. 11, p. 32-33)