Saturday, Dec. 28, will be the 40th birthday of one of the most significant pieces of environmental history in the U.S.: the Endangered Species Act. Now, with this landmark legislation entering its middle age, it’s an ideal time to reflect on where it has succeeded – and where it has fallen short.

“This landmark law has helped to stop the slide toward extinction of hundreds of species,” U.S. Secretary of the Interior Sally Jewell said in a statement. “Along the way, we have strengthened partnerships among states, tribes, local communities, private landowners and other stakeholders to find conservation solutions that work for both listed species and economic development.”

On balance, the ESA’s signature feature, the endangered species list, seems overwhelmingly positive. As of January 2013, the U.S. Fish and Wildlife Service has listed 2,054 species worldwide as endangered or threatened. Some 58 species have been taken off the list: 30 because they’ve recovered, 10 due to extinction, and 18 due to an error found in the original listing data. That might not seem like a resounding success on its face, but rebuilding the populations of species takes time – decades, often. In a 2012 report, the Center for Biological Diversity found that 90 percent of species covered by an ESA plan are recovering their numbers at the expected rate.

“Few laws of any kind can boast a 90 percent success rate,” the CBD says.

One of the most iconic successes of the ESA is the bald eagle. The bird that graces America’s national seal came near extinction in the 20th century, largely due to widespread use of the pesticide DDT. Eagles ate prey tainted with the pesticide, which then accumulated in the birds’ bodies. This accumulation of pesticide caused the eagles to lay eggs with weak shells, often breaking much too early for the chicks to survive. A ban on DDT and habitat protections helped the bald eagle population in the lower 48 states rebound from a low of around 400 breeding pairs in 1963 to a high of 9,789 pairs today. The bald eagle’s recovery was strong enough to merit it being taken off the endangered species list in 2007.

But criticisms abound. Getting on the endangered species list is no easy feat, with layer upon layer of bureaucracy and reviews and studies and comments and proposals. Dozens of plants and animals, such as the Amak Island song sparrow, are thought to have gone extinct while their cases were stuck in this legislative limbo. And many frustrated people may wonder why exactly projects that can bring jobs and enrich communities have to be put on hold to protect a stream for the snail darter.

Still, “the act helps humans more than it hurts,” the Wisconsin State Journal said in an editorial on Thursday. “Protecting the bald eagle, for example, led to a better understanding of and restrictions on the pesticide DDT – benefiting the regal birds as well as humans. An endangered mint [the scrub mint Dicerandra frutescens] has been found to act as a natural insecticide. The anti-cancer drug taxol comes from an endangered tree.”

(Technically, the Pacific yew, Taxus brevifolia, in which taxol was first discovered, is not endangered. It was thought to be threatened in the early 1990s -- before it was found to be useful, the tree was routinely destroyed in logging operations – but the tree was never actually listed as endangered by USFWS. However, other species of yew that have been harvested for taxol have been listed as endangered by the International Union for Conservation of Nature, which keeps a Red List of species separate from the ESA’s list).

That higher sentiment that man’s fate is intertwined with nature’s is what drove U.S. President Richard M. Nixon to first call for new endangered species protections back in the 1970s.

“While we share our environmental problems with all the people of the world, our industrial might, which has made us the leader among nations in terms of material well-being, also gives us the responsibility of dealing with environmental problems first among the nations,” President Nixon said in a February 1972 special message to Congress. “We can be proud that our solutions and our performance will become the measure for others climbing the ladder of aspirations and difficulties; we can set our sights on a standard that will lift their expectations of what man can do.”