Srila Prabhupada's class on Srimad Bhagavatam 7.9.38 given on the occasion of Gaura Purnima, Lord Caitanya's Appearance Day.
A transcript of the class can be found here: https://goo.gl/3xv7ks.
Srimad Bhagavatam 7.9.38
itthaṁ nṛ-tiryag-ṛṣi-deva-jhaṣāvatārair
lokān vibhāvayasi haṁsi jagat pratīpān
dharmaṁ mahā-puruṣa pāsi yugānuvṛttaṁ
channaḥ kalau yad abhavas tri-yugo ’tha sa tvam
In this way, my Lord, You appear in various incarnations as a human being, an animal, a great saint, a demigod, a fish or a tortoise, thus maintaining the entire creation in different planetary systems and killing the demoniac principles. According to the age, O my Lord, You protect the principles of religion. In the Age of Kali, however, You do not assert Yourself as the Supreme Personality of Godhead, and therefore You are known as Triyuga, or the Lord who appears in three yugas.