Law of the jungle.

Googled it. Got:

"Now this is the Law of the Jungle -- as old and as true as the sky;
And the Wolf that shall keep it may prosper, but the Wolf that shall break it must die.
As the creeper that girdles the tree-trunk the Law runneth forward and back --
For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack."

~The Law of the Jungle, Rudyard Kipling

From http://www.poetryloverspage.com/poets/kipling/law_of_jungle.html.