In this paper, we present a novel routing protocol for mobile delay tolerant networks, called Community-based Adaptive Spray (CAS). How to maximize routing performance (delivery ratio) and minimize resource consumption (number of message copies) is a common goal in these networks. Our protocol considers the following two aspects toward this goal: 1) selecting the intermediate nodes that are closest to the destination, based on their mobility patterns; 2) dynamically controlling the number of message copies according to the time-to-live of a message. Experiment results demonstrate that our protocol can improve routing performance and resource consumption compared to the state-of-the-art Spray-and-Wait and BUBBLE protocols.