THE EARTHWORM 287 



the average rate at which a layer of gravel or cinders would thus 

 settle if left undisturbed was something over two inches in ten 

 years. By weighing the castings brought up over a limited area 

 in a given period, he further estimated that in ten j^ars the bulk 

 of such material deposited on the surface would make a layer one 

 and one-half inches deep. The difference between the two esti- 

 mates he accounted for by the fact that not all the castings could 

 be collected and by the very conservative way in which the 

 second set of observations were made. A uniform layer of fine 

 soil one and one-half inches in diameter is equivalent, according 

 to these calculations, to one hundred and fifty tons of earth to 

 the acre in the ten-year period under consideration. Moreover, 

 such soil is deposited at the surface in a finely pulverized con- 

 dition and impregnated to some extent with waste products of the 

 worms' metabolic activities. The bulk of such material seems 

 not unreasonable, since it was conservatively estimated that 

 there were some 26,886 worms to the acre in the fields that were 

 being examined. In a garden plot, where it was estimated there 

 might be 53,767 worms to the acre, the effects would be propor- 

 tionately greater. 



Darwin's mental attitude in accumulating facts and in reflecting 

 upon their significance is nowhere better illustrated than in this 

 little volume on earthworms, which was based upon observations 

 extending over almost forty years. Other organisms that burrow 

 in the soil, like the ants and various insects, have similar effects, 

 and there are causes of a physical nature involved in the produc- 

 tion of the fine surface layers of the soil ; but the role of earthworms 

 in the locaUties where they abound cannot be questioned. 



Earthworms are nocturnal in their activities above ground, 

 although they sometim.es come to the surface in daylight following 

 a shower. During the daytime they he in the vertical part of 

 the burrow near the surface, unless engaged in excavating at a 

 greater depth. Often the mouth of the burrow is plugged with 

 bits of grass or other materials that have been drawn in at the 

 close of the night's activities. With the approach of darkness this 

 plug is pushed aside and the worm extends its body preparatory to 

 the nocturnal foraging. With the posterior end of the body 

 retaining its place within the burrow, the animal now explores the 

 surface in all directions. If disturbed, it can by a sudden contrac- 

 tion dash "like a rabbit into its burrow," as Darwin said, and thus 



