Recognizing the fossils as remains of once-living organisms, and arguing against the Deluge explanation, Leonardo reasoned that such fragile shells could not have been swept so far inland and survived intact. He also noted that the fossils commonly lay in successive rock layers, evidence that they were deposited by multiple events rather than by only one. And he observed that groups of different fossil shells found together resembled the living groups assembled in coastal waters. For all these reasons, Leonardo correctly concluded that the fossils came from animals which once inhabited an ancient sea that covered the land.