This optimistic, soft-focus fantasy of de-extinction, while thrilling to Ben Novak, is disturbing to many conservation biologists, who consider it a threat to their entire discipline and even to the environmental movement. At a recent Revive & Restore conference and in articles appearing in both the popular and academic press since then, they have articulated their litany of criticisms at an increasingly high pitch. In response, particularly in recent months, supporters of de-extinction have more aggressively begun to advance their counterarguments. “We have answers for every question,” Novak told me. “We’ve been thinking about this a long time.”

The first question posed by conservationists addresses the logic of bringing back an animal whose native habitat has disappeared. Why go through all the trouble just to have the animal go extinct all over again? While this criticism is valid for some species, the passenger pigeon should be especially well suited to survive in new habitats, because it had no specific native habitat to begin with. It was an opportunistic eater, devouring a wide range of nuts and acorns and flying wherever there was food.

There is also anxiety about disease. “Pathogens in the environment are constantly evolving, and animals are developing new immune systems,” said Doug Armstrong, a conservation biologist in New Zealand who studies the reintroduction of species. “If you recreate a species genetically and release it, and that genotype is based on a bird from a 100-year-old environment, you probably will increase risk.” A revived passenger pigeon might be a vector for modern diseases. But this concern, said David Haussler, the co-founder of the Genome 10K Project, is overblown. “There’s always this fear that somehow, if we do it, we’re going to accidentally make something horrible, because only nature can really do it right. But nature is totally random. Nature makes monsters. Nature makes threats. Many of the things that are most threatening to us are a product of nature. Revive & Restore is not going to tip the balance in any way.” (Some scientists have speculated that, by competing for acorns with rodents and deer, the passenger pigeon could bring about a decrease in Lyme disease.)

More pressing to conservationists is a practical anxiety: Money.