The present findings apply to a particular area, the four northern counties of England, and must be viewed within the limits of the data presented.

Only one tick, Ixodes ricinus L., the so-called sheep tick, infests sheep and cattle in northern England. I. canisuga Johnston, alone or with I. ricinus, has been found on sheep dogs and foxes.

The distribution of the sheep tick in this region is markedly discontinuous, the total infested areas including only about one-fifth of the hilly country, to which the tick is almost exclusively confined.

In north-west Northumberland, tick distribution shows a significant degree of correlation with certain factors inherent in or influenced by the surface geology. Broadly speaking, where surface geology lends itself to relatively good natural drainage and soil, the grazing is relatively good and ticks are absent; and where surface geology results in relatively poor natural drainage and/or soil, the grazing is relatively poor (‘rough’) and ticks are present.

No consistent correlations between soil characteristics—i.e. pH, available phosphates and potash, soil textures, mechanical analysis and soil depth—and tick distribution have been found. Nor is there a consistent correlation- between quality of natural drainage and tick distribution. The interaction of soil and natural drainage factors, however, profoundly influences the character of the vegetation layer.

In the four northern counties, the same plant dominants occur on tick-infested and tick-free hill lands. Where grazing is rough ticks are usually present irrespective of whether the dominant plant is one of the rough grasses (Nardus, Molinia, Agrostis, Aira), bracken or heather; where grazing is relatively good (i.e. not rough), ticks are invariably absent.

Observations on a smaller scale show that the thicker, i.e. rougher, the vegetation layer, the denser the tick population and also the thicker the vegetation layer, the thicker the basal mat. On five plots it has been shown that there is a consistent positive correlation between mat thickness and tick population density: the thicker the mat,’ the denser the tick population irrespective of whether bracken or grass is the dominant. Thus the chief controlling factor in tick distribution is the physical character of the vegetation layer.