The hind limbs which were useful on land have been eliminated and all that persists are vestigial bones or cartilages which are buried deep below the surface of the body. The forelimbs have undergone reduction and modification into flippers which assist in the turning and diving. The flippers are useful in other ways, providing a platform on which the baby may stay when danger threatens. They are also useful during courtship and mating, but not for combat. The toothless whales do not have too much to fight with. They may strike an adversary with the powerful tail flukes, and during courtship the males jostle and bump each other. Whales are almost completely hairless, save for a few bristles on their heads. Certainly the elimination of hair has improved streamlining, and has reduced the frictional drag. Furthermore, continuously wet hair could not have been of much value in keeping the whale warm. It is also possible that a hairy whale would have been very much bothered by skin parasites which would have flourished in the quiet water between the hairs. However, if this prompted the loss of hair, it was in vain for now the streamlined bodies of humpbacks and right whales are marred by large encrusting barnacles. It is surprising that the barnacles do not completely cover the whales. Perhaps they are scraped off on the bottom, or they cannot flourish during the long migration or in plankton-impoverished waters of the winter quarters. At any rate the parasites are kept partially under control so that much of the streamlined surface is unblemished. If the physical properties of water forced upon whales a common shape, they did at least, by the buoyant effect, free the animals of the need for structural and muscular developments to support themselves against the pull of gravity. Free of this structural problem, whales were able to evolve into the largest mammals which the world has ever known. As they became larger, they had to shift in their feeding to slower and less maneuverable prey. It would appear that the porpoises, which feed on the rapid-swimming, elusive fishes, are small in order to catch their prey. The whales, which have specialized to feed on the jet propelled squids, were able to evolve into much larger whales because they could capture the squid either by stealthy approach or by sucking the squid into the mouth, thus counteracting its jet. [Illustration: Tailflukes extended for deep dive. _Courtesy Scripps Institute of Oceanography._] Whale Types _Porpoises and Dolphins_