88 THE ZOOLOGIST. 



progression cause the water behind the Killer, for instance, to 

 be violently churned as from the screw of a powerful steamer, 

 are all provided for in the smallest possible compass. 



The posterior extremity of the body of the Cetacean is fur- 

 nished with the flukes, which consist of strong fibrous tissue 

 covered with skin — in some forms, symmetrically fimbriated — 

 and with which certain of the muscles just mentioned are con- 

 nected. The "blubber" again becomes very thin as it ap- 

 proaches the root of the tail, and finally is lost in the fibrous 

 substance of the organ. The strength of the muscles moving 

 the tail enables the animal to strike the water with great power 

 vertically and at any angle. The larger toothed forms — such 

 as the Sperm Whale — crush strong whale-boats into fragments 

 with a single blow, or occasionally send a harpooner through the 

 air to a distance, and the Californian Grey Whale was equally 

 dextrous with its tail. By its aid also these huge animals 

 spring clear of the water with all the agility of a Salmon, a 

 feature likewise very characteristic of the smaller forms, such 

 as Dolphins, groups of which now and then leap from the 

 slopes of the great waves in the North Sea, making a loud noise 

 as they again strike the water. 



Two prominent views are held with regard to the homologies 

 of the caudal flukes. Thus Owen, Flower, Huxley, Glaus, and 

 Parker suppose that the whole hind limb has been suppressed 

 or atrophied externally, and that the flukes, like the dorsal fin, 

 have been secondarily acquired. Eyder, again, thinks that the 

 flukes are probably degenerate homologues of hind feet ; while 

 Gray and even earlier authors held that they represent the 

 whole hind limb. Eyder supports his view by pointing out 

 that the skeletal elements of the posterior limb have been 

 atrophied from without inward, and that only traces of the 

 femur and pelvic arch remain in certain forms. Further, that 

 the muscles of the tail are connected with the flukes, the blood- 

 vessels of which are arranged in a dorsal and in a plantar set 

 as in the foot ; that there is a tendency in the nerves of the 

 lumbar plexus to be prolonged toward the tail to supply caudal 

 muscles; that the direction of the crus, when developed, is in 

 the line of the flukes ; and, lastly, that the flukes are developed as 

 in other limbs. Ryder is supported by Prof. Gill, who also con- 



