October 1, 1891.] 



KNOWLEDGE. 



185 



be smoothed over remains to be seen, but the spirited 

 action of our Gallic neighbours ensures that in one direc- 

 tion at least international yacht racing is not altogether 

 a thing of the past. 



SWIMMING ANIMALS. 



By E. Lydekker, B.A.Cantab. 

 {('ontimu'd from pai/c 168.) 



THE Divers, Auks, and Grebes are, with the exception 

 of the Penguins, those birds which appear to have 

 been most profoundly modified for a life in the water, 

 being equally at home both on and below its sur- 

 face. In these birds the short legs are placed so far 

 back that when on land the body is carried in a more or less 

 nearly erect position, as we may observe in the Guillemots and 

 PaiBns of our coasts. Although the legs themselves are very 

 short, yet the toes are elongated, so as to convert the feet 

 into very powerful oars. Most oi these birds look exceeding 

 awkward when on laud, and as they use both their feet and 

 wings in diving, the water is undoubtedly the element in 

 which they are most at home. Speaking of the Eed- 

 Throated Diver of Northern Europe, Mr. Dresser observes 

 that " it swims low down in the water, and when uneasy 

 or alarmed will submerge its body below the surface, 

 leaving only the head and neck in view. When it dives it 

 vanishes beneath the surface without noise or flutter, and 

 propels itself along with its wings as well as its feet, 

 frequently remaining for some time before it emerges to 

 view again." 



The most remarkable modification which birds have 

 undei'gone for the purposes of an aquatic life is, however, 

 presented by the Penguins of the Southern Ocean. These 

 grotesque birds, some of which attain a very large size, are 

 even more upright than tlie Puflins, and when arranged in 

 lines on the clitt's of the Antarctic lands have been compared 



Fio. 7. — Tub Common Seai. 



to regiments of soldiers. Their short wings, which are of 

 course utterly useless for flight, and have but a very 

 limited range of motion, are converted into flipper-like 

 paddles, covered with short bristly feathers, their only use 

 being as additional swimming organs. We have here, 

 therefore, an instance of an organ originally modified for 

 an especial purpose — flight — subsequently undergoing a 

 kind of retrograde modification for a totally different use, 

 although still retaining the structiu-al peculiarities which it 

 presents in ordinary birds. Certain features in the structure 

 of the leg of the Penguins suggest, however, that these birds 

 belong to a very primitive type. 



We must not conclude our notice of swimming birds with- 

 out reference to the extinct Hesperuntis, of the Cretaceous 

 beds of the United States. This remarkable bird, which 

 was nearly six feet in length, shows evidence of its 

 relationship to reptiles by the retention of a complete 

 series of sharp-pointed teeth in both jaws. In the 

 structure of its bones it appears to come nearest to the 

 Grebes and Divers, but it differs from all the swimming 

 birds in having lost (so far as can be determined) all 

 traces of wings ; and thus affords one of several instances 

 where species, long extinct, are in certain respects more 

 specialized than any of their living relatives. 



Our remaining examples of Swimming Animals are 

 taken from the class of Mammals, or Quadrupeds, as they 

 are often popularly, though inconveniently, termed. And 

 we shall find that in certain members of this group the 

 adaptation to an aquatic life has been so complete as to 

 have led to the loss of all external features characteristic 

 of ordinary members of the class, and has thus induced 

 the erroneous popular belief that the animals in question 

 are really fishes. 



In several groups of Mammals we find that a few 

 species, or genera, have been more or less modified so as 

 to become export swimmers and divers. Instances of 

 these are afl'orded by the Australian Duck-Bill (Knowledge, 

 isflO, p. 84) among the egg-laying ^Mammals, the Otter 



among the Carni- 

 vores, the Beaver 

 and Water- Vole 

 among the Ro- 

 dents, and the 

 Hippopotamus 

 among the Ungu- 

 lates. Since, how- 

 ever, none of these 

 _^ • depart very widely 



from the normal 

 type of structure, 

 we may pass td 

 the consideration 

 of two groups, in 

 which all the 

 members have 

 undergone more 

 or less profound 

 structural altera- 

 tions solely and 

 simply for the 

 purpose of swim- 

 ming. 



The first of these 

 groups is that of 

 the Seal s and Wal- 

 ruses, which form 

 a special division 

 of the Carnivorosi 

 Through the spfr 



