EVOLUTION AND EXTERNAL APPEARANCE 



69 



Figure 31. Relative position at 

 the surface of the water due to 

 gravity and upthrust of (a) 

 a horse and (b) a dolphin. 



case to the tip of the head where together they constitute the blowhole 

 (see Fig. 33). This is probably best explained by the fact that, the more 

 Cetaceans became adapted to their aquatic mode of life, the more 

 essential it became for the nostrils to migrate to their present posterior 

 and superior position in the head. This must have happened to the 

 ancestors of the Sperm Whale as well, but as the large spermaceti case 

 developed and filled with light material, the nostrils were obviously in the 

 wrong place. Now there is a biological law, Dollo's 'law of irreversible 

 evolution', which states that evolution never retraces its steps. Hence, 

 once the nasal passage of the Sperm Whale had migrated to its present 

 position, it could not return to its original situation, and a new solution 

 had to be found : the elongated passage through the spermaceti case. 

 Further evidence for this assumption is found in the position of the nasal 



