וְנָשַׁל֩ יְהוָ֨ה אֱלֹהֶ֜יךָ אֶת־הַגּוֹיִ֥ם הָאֵ֛ל מִפָּנֶ֖יךָ מְעַ֣ט מְעָ֑ט לֹ֤א תוּכַל֙ כַּלֹּתָ֣ם מַהֵ֔ר פֶּן־תִּרְבֶּ֥ה עָלֶ֖יךָ חַיַּ֥ת הַשָּׂדֶֽה׃
And the LORD thy God will cast out those nations before thee by little and little; thou mayest not consume them quickly, lest the beasts of the field increase upon thee.
Read midrashic interpretation of Deuteronomy 7:22: and classical rabbinic exegesis.