Chapter Seven 



J_N MOST mammals the head and neck can develop partly independent 

 of the trunk and the two together may in some respects be compared to 

 an appendage such as the arm or leg. The neck may, and usually does, 

 become long or short according to leg length, so that the mouth may 

 reach the ground ; and it almost invariably shortens in mammals which 

 can employ the fore foot as a hand for conveying food to the mouth. 

 The neck must have strength according to size of head and the sort of 

 work that the latter performs, and agility according to the needs of the 

 animal. In the case of a mammal the size of a rat no more need be 

 said, for anything of this size and such light weight could doubtless run 

 as fast and easily were its neck twice as long. But in a large body weigh- 

 ing hundreds of pounds another and vital element is added, constituting 

 adequate equilibration according to the manner of locomotion habitually 

 employed, and this introduces many complex factors which we can only 

 partially understand. In the case of such an amphibious mammal as the 

 sea-lion, the head and neck must act as a balancer, in a gravitational 

 sense, during terrestrial progression. This no longer figures in a mam- 

 mal that is exclusively aquatic. In such the head and neck usually must 

 be of some particular mass and length either so as efficiently to play an 

 important part in swimming, or more rarely the method of progression 

 is such that the anterior end of the animal plays no important part in 

 it, and this portion of the body can then develop in response to some 

 other stimulus. 



The conditions which aquatic reptiles seem to have encountered are 

 so at variance with those that have applied to mammals that separate 

 though brief mention should be made of them. Most of the large, 

 extinct, aquatic reptiles seem to have progressed for ages after taking 

 to the water by a rhythmic, diagonal movement of all four limbs that 

 may be compared to the trot, so that the head and neck had no need to 

 act in maintaining equilibrium, or as a rudder. Hence, while this mode 

 of swimming is employed the neck may lengthen or shorten in response 

 to other stimuli. Further, the neck in the two classes can hardly be 

 compared, for mammals exhibit a truly extraordinary conservatism in 



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