Hubert M. Morris 297 



species such as Trichocera and Chironomidae larvae, which probably are 

 of some service in helping to open up the soil. 



11. Distribution in Depth. 



The depth at which the different organisms occurred was of consider- 

 able interest, and the samples were taken in five separate layers in order 

 that their distribution might be accurately determined. This distribution 

 was considerably affected by the ploughing of the plots, but seemed to 

 be very little influenced by the operations of cultivation, harrowing and 

 drilling. 



In taking a sample of soil it was usually quite clear to what depth 

 the ploughing had affected the soil, and as a rule a distinct change in 

 the character of the soil was noticed in the fourth layer, taken between 

 the five and seven-inch levels. 



Of the total number of insects present, taking the whole period of 

 the investigation, in plot 2, 78-7 per cent., and in plot 3, 50-3 per cent., 

 occurred in the first two layers of soil, that is, between the surface and 

 a depth of three inches. The percentages at the different depths were, 

 for the manured plot: I 51-5; II 27-2; III 11-0; IV 6-4; V 3-8; and for 

 the control plot: I 25-3; II 25-0; III 33-0; IV IM; V 5-5. Taking only 

 the period from the commencement of the investigation in February 

 until the plots were ploughed on October 13th, the percentages at the 

 different depths were, for the manured plot: I 58-0; II 27-7; III 9-6; 



IV 2-5; V 2-2, and for the control plot: I 26-0; II 25-0; III 33-9; IV 10-2; 



V 4'8. Similarly, from the time of ploughing to the end of the investiga- 

 tion (October to January), the percentages at the different depths were, 

 for the manured plot: I 8-7; II 24-3; III 20-0; IV 32-3; V 15-0, and for 

 the control plot: I 16-6; II 24-0; III 22-2; IV 22-2; V 14-8. 



It must be borne in mind, in comparing the percentages in the upper- 

 most layer with those in the other layers, that the volume of soil in this 

 top layer was considerably less than in the other layers, as it consisted 

 of the soil between the surface and a depth of one inch below the surface 

 only, while the remaining layers consisted of the soil for a depth of two 

 inches. 



Most groups of insects, etc., considering the period of the investiga- 

 tion as a whole, occurred in the largest numbers in the second layer, 

 with a rather lower percentage in the first. The third usually contained 

 a distinctly smaller percentage than the second, quite commonly being 

 from one-half to one-third the number, w^hile the fourth layer usually 

 stood in about the same relation to the third, the difference being in 



