INTRODUCTION 



mammal may be in a straight line with slight deviation, while in the 

 case of another, perhaps handicapped by some bodily equipment less 

 readily adaptable to aquatic requirements, there may be much trial and 

 error, with the employment of temporary makeshifts. Thus one pair 

 of limbs may become quite highly modified for propulsion, but yet 

 incapable, because of some mechanical defect in the method of employ- 

 ment, of developing as high speed as can the altered tail, in which case 

 the latter is apt to take over the duty of locomotion and the former fall 

 into disuse. But this is so only up to a certain point, and the "law 

 of irreversible evolution" is believed to prevent the redevelopment of 

 a part once atrophy is very definitely under way. The sea-lion might 

 develop its muscles so that the hind limbs would become the chief or- 

 gan of propulsion, but whales have lost their hind limbs forever. 



Thorough modification for an aquatic life by a mammal involves 

 not only more numerous but more profound anatomical and physiological 

 alteration than for any other sort of existence. The volant specialization 

 of the bat has resulted in considerable change; but this has concerned 

 only lengthening or rotation of bones, development of membranes, and 

 not particularly profound muscular modification. In the whale, how- 

 ever, the evolution of bodily details has been truly astonishing, and to 

 only a slightly lesser extent in sirenians. Thus in mammals the aquatic 

 life is the only one that has resulted in the complete loss of the external 

 portions of the hind limb, acquisition of supernumerary phalanges to 

 the digits, migration of the nostrils to the top of the head, and entire 

 inability to travel for at least a few yards when placed on land. The 

 entire specialized development has been confined to the ends of securing 

 food and of locomotion involving but a single pair of major movements. 

 In some sorts the former specialization does not have much effect upon 

 body form, so that as far as concerns external conformation the only 

 major stimulus has been a simplified propulsive one. This being the 

 case it should not cause undue astonishment that the most perfect degree 

 of highly specialized convergence in bodily form to be found between 

 different vertebrate classes is furnished by some species of shark, ichthyo- 

 saur, and whale (frontispiece) . 



But while the external form, and even some of the internal anatomy, 

 of a whale may be said to be simplified, internal alteration from the 

 normal mammalian sort is often so profound in both quantity and 

 quality that one may be at a complete loss to interpret it correctly. And 

 yet all organs of the terrestrial mammal are present, even though they 

 may be vestigeal. The change from what we consider as the mammalian 



[5] 



