The biodiversity crisis characterized by large scale species endangerment is a serious problem faced by modern humankind. Species extinction is an unavoidable natural phenomenon and cannot be prevented. Species on earth are in a constant birth and death evolutionary process: old species die off or evolve into new ones while new species evolve to fill the empty niches left behind by extinct taxa. Causes of species extinction may be attributed to maladaptation of the species to the changed environment, or a species may be simply wiped out by a changed environment or bad luck in evolution; or even due to stochastic reasons, independent of the species adaptability to the environment. Like the other life forms, a species has its life span. Average life span for a species is less than 10 million years, spanning from one million years to several million years depending on the taxon. About 4 billion species once lived on earth, but 99% of them are already extinct. Species extinction is meaningful issue in evolution, if all species had persisted on the earth today, the earth would have been too crowded for the limited living space and ecological niches. The evolution of life would have stopped; new life forms would have no chance to emerge. Each time after a mass species extinction, the earths biodiversity rebounded, marked by arising of new species, genera and families. For example, the rapid radiation of mammals on earth after extinction of dinosaurs, ungulates and Homindae quickly occupied the empty niches left by dinosaurs. Humankind is now an important selection force in evolution. Five major mass extinctions occurred on earth before human civilization. Ample evidence indicated that we now confronting the Sixth Mass Extinction: since the Industrial Revolution, hundreds of species already disappeared from earth. Led by the amphibians, many mammals, birds, and reptile and plants on the earth are endangered. Besides the biological evolutionary cues, loss of habitat, overexploitation, environmental pollution, plus climate change, and invasive species are the main causes that threaten survival of extant species. The extinction rate in modern times is higher than the historical background records. Endangered species could be classified as evolutionarily endangered species or ecologically endangered species, the endangerment of the former is characterized by phylogenetic, endemic and life history causes whereas the latter are endangered by human induced environmental changes in ecological time. Human activities modify or change the environment and threaten the survival of many species; Human activities are the main driving force behind the Sixth Mass Extinction. How do we save those endangered species? It is better for the doer to undo what he has done. Humankind has an inescapable responsibility to rescue those species in peril. We should take measures to avoid the high extinction risks in modern fauna and flora.