Table XIV. represents a numerical estimate of the invertebrate fauna of two plots of arable land at Rothamsted. The soil is clay with flints overlying chalk, and the land in question has been devoted for eighty years to continuous cropping with wheat; one plot (No. 3) receives an annual dressing of farmyard manure at the rate of 14 tons per acre, and the other plot (No. 2) receives no natural or artificial fertilizer. The significant feature in a comparison of the fauna of the two plots is the great numerical increase in organisms due to the addition of manure. From the point of view of distribution in depth, Fig. 20 clearly demonstrates that the bulk of the fauna is concentrated in the first three inches of the soil. With the exception of the _Acarina_ it is evident that the limits of vertical distribution extend below the depth of nine inches investigated, although the numbers of organisms likely to be present are inconsiderable. The _Oligochæta_, or true earthworms, occur in Rothamsted soil in numbers very much in excess of the figures given by Darwin, who quoted observations by Hensen. The latter authority calculated that there were 53,767 earthworms in an acre of garden soil, and estimated that about half that number would be present in an acre of corn field. In the Rothamsted investigations their numbers exceeded Hensen’s estimate over 16 times in unmanured land, and over 36 times in manured land. In an area of pasture-land in Cheshire few insects occurred below a depth of 2 inches, and they reached the limit of their vertical distribution at or near 6 inches. Their number (3,586,000 per acre) is considerably in excess of that present in unmanured arable land at Rothamsted. [Illustration: 1, _Collembola_; 2, _Thysanura_; 3, _Orthoptera_; 4, _Thysanoptera_; 5, _Hemiptera_; 6, _Lepidoptera_; 7, _Coleoptera_; 8, _Diptera_; 9, _Hymenoptera_. FIG. 21.--Number of individuals in the different orders of insects in manured and unmanured arable land at Rothamsted. (From Morris, “Annals of Applied Biology,” vol. ix., nos. 3 and 4, Cambridge University Press.)] DOMINANCE OF CERTAIN SPECIES AND GROUPS. In Fig. 21 a numerical analysis is given of the different orders of insects represented in Rothamsted soil. The ascendency of the _Hymenoptera_ and _Collembola_ is almost entirely due to the occurrence of three species in large numbers, viz., the ant _Myrmica lævinodis_ and the _Collembola_, _Onychiurus ambulans_ and _O. fimetarius_. In the unmanured plot these two _Collembola_ constituted 13 per cent. of the insects and the species of ant accounted for nearly 28 per cent. In the manured plot they amounted respectively to 27 per cent. and 36 per cent. of the insects present. Next in order of numerical ascendency are larval _Diptera_, mainly belonging to the families _Cecidomyidæ_, _Chironomidæ_, and _Mycetophilidæ_. The _Diptera_ are followed by the _Coleoptera_, whose most abundant representatives are larval _Elateridæ_ (wireworms).