Mammals to Aquatic Life. 161 



length and darkly pigmented on the upper jaw, but pale on 

 the lower, sprang not directly from the surface of the skin, 

 but were sunk in deeply pigmented pits. Such pits were 

 moreover to be found spread over the body as far as the 

 middle and especially numerous on the back, yet without any 

 hairs springing from them. Some were deeper, some shal- 

 lower, some merely indicated by a spot of pigment. We are 

 therefore entitled to say that the last vestiges of a hairy coat 

 are found on the entire anterior half of the body in the adult 

 rorqual. 



In the whalebone whales the remains of a former covering 

 of hair are still quite distinct ; in this respect a sharp contrast 

 is afforded by the toothed whales, which (with tlie exception 

 of Inia) possess no hairs in the adult state, and in embryonic 

 life only a few tactile hairs on both sides of the upper lip. 

 That even these may be absent is proved by the white whale 

 and the narwhal, in which not even traces of hairs are found 

 at any period of their development. 



Throughout the series of aquatic mammals we have learnt 

 to recognize the reduction of the hairy coat as a striking 

 phenomenon of adaptation, and have been able to follow its 

 disappearance step by step both with the aid of comparative 

 anatomy and also developmentally. The more the animals 

 develop from temporary into permanent inhabitants of the 

 water, of less use does the coat of hair become ; owing to the 

 change of medium and consequent better conduction of heat, 

 it is no longer sufficient to protect the body from loss of 

 warmth, and it is superseded by the layer of blubber beneath 

 the skin. 



The further phenomena of adaptation as seen in the skin 

 are of a more secondary nature ; integumentary glands, smooth 

 musculature, and integumentary nerves undergo gradual 

 reduction, ending in complete disappearance. 



Now in wishing to institute investigations into the relation- 

 ships of these animals we must not adduce the similarities to 

 be found in the structure of the integument as proofs of their 

 affinity ; for these converging resemblances have arisen inde- 

 pendently of one another. The influence of the aquatic life 

 on the integument manifests itself in the same direction. 

 Only those features can be used as phyiogenetic connecting- 

 links which have persisted independently of these influences. 



Still confining our attention to the whales, we at once find 

 a marked difference witiiin the order, in that the bearded 

 ■whales throughout exhibit a relatively much richer covering 

 of hair than the toothed whales. The mode of life of both is 

 precisely similar, and we must therefore conclude that the 



