SB 1.15.25-26

jalaukasāṁ jale yadvan
 mahānto ’danty aṇīyasaḥ
durbalān balino rājan
 mahānto balino mithaḥ
evaṁ baliṣṭhair yadubhir
 mahadbhir itarān vibhuḥ
yadūn yadubhir anyonyaṁ
 bhū-bhārān sañjahāra ha
Word for word: 
jalaukasām — of the aquatics; jale — in the water; yadvat — as it is; mahāntaḥ — the larger one; adanti — swallows; aṇīyasaḥ — smaller ones; durbalān — the weak; balinaḥ — the stronger; rājan — O King; mahāntaḥ — the strongest; balinaḥ — less strong; mithaḥ — in a duel; evam — thus; baliṣṭhaiḥ — by the strongest; yadubhiḥ — by the descendants of Yadu; mahadbhiḥ — one who has greater strength; itarān — the common ones; vibhuḥ — the Supreme Personality of Godhead; yadūn — all the Yadus; yadubhiḥ — by the Yadus; anyonyam — among one another; bhū-bhārān — the burden of the world; sañjahāra — has unloaded; ha — in the past.
Translation: 
O King, as in the ocean the bigger and stronger aquatics swallow up the smaller and weaker ones, so also the Supreme Personality of Godhead, to lighten the burden of the earth, has engaged the stronger Yadu to kill the weaker, and the bigger Yadu to kill the smaller.
Purport: 

In the material world the struggle for existence and survival of the fittest are laws because in the material world there is disparity between conditioned souls due to everyone’s desire to lord it over the material resources. This very mentality of lording it over the material nature is the root cause of conditioned life. And to give facility to such imitation lords, the illusory energy of the Lord has created a disparity between conditioned living beings by creating the stronger and the weaker in every species of life. The mentality of lording it over the material nature and the creation has naturally created a disparity and therefore the law of struggle for existence. In the spiritual world there is no such disparity, nor is there such a struggle for existence. In the spiritual world there is no struggle for existence because everyone there exists eternally. There is no disparity because everyone wants to render service to the Supreme Lord, and no one wants to imitate the Lord in becoming the beneficiary. The Lord, being creator of everything, including the living beings, factually is the proprietor and enjoyer of everything that be, but in the material world, by the spell of māyā, or illusion, this eternal relation with the Supreme Personality of Godhead is forgotten, and so the living being is conditioned under the law of struggle for existence and survival of the fittest.