In the Brahma-samhita also it is said indirectly that although there are many incarnations of the Lord, such as Rama, Nrsimha, Varaha, Matsya, Kurma and many others, the Lord Himself sometimes incarnates in person. Lord Krsna and Lord Sri Caitanya Mahaprabhu are not, therefore, incarnations, but the original source of all other incarnations. So the Lord is the inexhaustible source for innumerable incarnations which are not always mentioned. But such incarnations are distinguished by specific extraordinary feats which are impossible to be performed by any living being. That is the general test to identify an incarnation of the Lord, directly and indirectly empowered.
Some incarnations mentioned above are almost plenary portions.
For instance, the Kumaras are empowered with transcendental knowledge. Sri Narada is empowered with devotional service. Maharaja Prthu is an empowered incarnation with executive function. The Matsya incarnation is directly a plenary portion. So the innumerable incarnations of the Lord are manifested all over the universes constantly, without cessation, as water flows constantly from waterfalls.