by Charles Hirshberg

About the author: Charles Hirshberg is a staff writer for Life magazine.

Once upon a time, not so long ago, zoos were little more than jails. Animals were kidnapped from the wild and imprisoned in bleak cells. Inbreeding was so common that their offspring rarely survived.

Today, zoos are being reinvented, revolutionized. Old cages are being knocked down and replaced with lush habitats. And thanks to an ingenious breeding program, baby animals are busting out all over.

Kejana and Baraka are two bright-eyed young gorillas, full of curiosity and mischief. As Lisa Stevens watches them bound from branch to branch in Washington, D.C.’s National Zoo, she shakes her head: “I cannot believe how far we’ve come.”

Stevens, curator of the zoo’s primates (except, of course, its...