FREE BOOKS

Author's List




PREV.   NEXT  
|<   41   42   43   44   45   46   47   48   49   50   51   52   53   54   55   56   57   58   59   60   61   62   63   64   65  
66   67   68   69   70   71   72   73   74   75   76   77   78   79   80   81   82   83   84   85   86   87   88   89   90   >>   >|  
s required for terrestrial locomotion becoming better adapted for aquatic locomotion, and the whole outline of the animal more fish-like in shape. This is the stage which we actually observe in the seals, where the hind legs, although retaining all their typical bones, have become shortened up almost to rudiments, and directed backwards, so as to be of no use for walking, while serving to complete the fish-like taper of the body. (Fig. 2.) But in the whales the modification has gone further than this so that the hind legs have ceased to be apparent externally, and are only represented internally--and even this only in some species--by remnants so rudimentary that it is difficult to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-like in shape. (Fig. 3.) But profound as are these alterations, they affect only those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in a fin-shaped sack, so as to render them useless for any purpose other than swimming (Fig. 4.) Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relations to one another; but modified in form so as to offer the least possible resistance to the water. In short, it may be said that all the modifications have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life. [Illustration: FIG. 2.--Skeleton of Seal, 1/8 nat. size. Drawn from nature (_R. Coll. Surg. Mus._).] [Illustration: FIG. 3.--Skeleton of Greenland Whale, 1/100 nat. size. The rudimentary bones of the pelvis are shown on a larger scale in the upper drawing. (From Prof. Flower.)] [Illustration: FIG. 4.--Paddle of Whale compared with Hand of Man. Drawn from nature (_R. Coll. Surg. Mus._).] Now I have chosen the case of the whale and porpoise group, because they offer so extreme an example of profound modification of structure in adaptation to changed conditions of life. But the same thing may be seen in hundreds and hundreds of other cases. For inst
PREV.   NEXT  
|<   41   42   43   44   45   46   47   48   49   50   51   52   53   54   55   56   57   58   59   60   61   62   63   64   65  
66   67   68   69   70   71   72   73   74   75   76   77   78   79   80   81   82   83   84   85   86   87   88   89   90   >>   >|  



Top keywords:

Illustration

 

aquatic

 

nature

 

rudimentary

 
mammalian
 

profound

 

organism

 

Skeleton

 

adaptation

 

retains


hundreds

 

locomotion

 

typical

 
adapted
 
modification
 
extreme
 

structure

 

divergence

 

conditions

 

effected


changed

 

modified

 

relations

 
resistance
 

modifications

 

drawing

 
Paddle
 
Flower
 

Greenland

 
anatomical

larger
 

pelvis

 
compared
 

purely

 
porpoise
 

securing

 

perfect

 
chosen
 

compatible

 

existence


serving

 
complete
 

walking

 

directed

 
backwards
 

whales

 

represented

 

internally

 
externally
 

apparent