54 EARTHWORMS AND THEIR ALLIES [ch. 



an entirely subterranean existence ; but as was pointed 

 out in chapter i these exceptions are but few and 

 the immense bulk of earthworms fully justify their 

 name. 



Nevertheless there are many arguments which 

 tend to show that these purely land-dwellers have 

 grown out of exclusively water-dwellers and even 

 that the change from the one mode of life to the 

 other has been accomplished comparatively recently. 

 For there are here and there vestiges of structures 

 which seem only fitted for an aquatic life; and in 

 other cases structural changes have commenced 

 which would appear to be in definite relation to the 

 underground mode of life prevalent to-day. Let us 

 consider for a moment the differences which obtain 

 between the conditions of life in water or in soft mud 

 at the bottom of pool or river, and those which are 

 undergone by a dweller in stiff soil or vegetable 

 debris. In the first case the medium is fluid or at 

 most very soft, while the soil is at least stiffer and 

 harder to traverse. 



Secondly the transition between the very bottom 

 of a pool and the top layers of the water is more or 

 less gradual, while the stiff soil ends abruptly in the 

 tenuity of the atmosphere. 



A third point of difference is doubtless the smaller 

 supply of readily available oxygen in the still pools 

 and even rapid rivers, which in certain stagnant pools 



