In what researchers are saying could be a big advance for endangered species, a house cat gave birth three weeks ago to a rare African wildcat. Scientists at the Audubon Institute Center for Research of Endangered Species in New Orleans said that they had transferred a frozen embryo between species, and that the house cat, after giving birth to the wildcat, was now nursing and protecting it as if it were her offspring. Ron Foreman, chief executive of the institute, said, “If extinction happens in the wild, the technology will be there to bring the species back.” However, Rebecca Spindler of the National Zoo’s Conservation and Research Center cautioned that the process used by the New Orleans scientists was no substitute for conservation.