SCIENCE-GOSSIP. 



41 



the presence of certain micro-organisms. For the 

 field-naturalist, however, a fertile and highly 

 cultivated tract is apt to present less of interest 

 than a wilder or more barren region in which 

 nature is seen under a more primitive aspect. 



The deeper structure of the ground, the " solid 

 geology," as it is called, is the factor which has 

 had the chief influence in moulding the contour of 

 the surface. The harder strata, which present the 

 greater resistance to denudation, have, in the 

 course of ages, been left standing out in relief as 

 hills and headlands, while the softer beds have 

 been worn out into valleys and bays. In the 

 south-east of England, as already mentioned, we 

 have no old crystalline or igneous rocks, and no 

 hard limestones, nor have we mountains on which 

 to find an Alpine flora or fauna. The chalk is the 

 dominant feature of our area. It exists, or has 

 existed, over the whole region, either on the 

 surface or covered by newer beds. Being of great 

 thickness, with comparative hardness and resist- 

 ance to denudation, it forms a series of hill ranges, 

 on which plants and animals of kinds which 

 prefer a calcareous soil abound. The other hill 

 ranges in our area are formed by the silicious beds 

 of the Lower Greensand, Hastings sand, or tertiary 

 gravels. It is on these latter ranges that the few 

 plants of northern or mountain type which occur 

 in our area are met with, e.g., the club-moss 

 {Lycopodium clavatmn), at Leith Hill, Holtye 

 Common and Tilgate Forest. 



The rainfall increases, caeteris paribus, with the 

 elevation, and the moist air of these wooded hills is 

 favourable to the growth of cryptogams, such as 

 ferns, mosses, lichens and fungi. These are far 

 more plentiful in those situations than in lower and 

 drier regions, and more especially than in such as 

 are exposed to the deleterious influence of town 

 smoke. The existence of hard strata is also 

 necessary for the formation of steep, rocky sites, 

 suitable for the habitat of rock-loving plants. 

 Hence, except on the sea coast, where we have fine 

 chalk cliffs — at Dover, Beachy Head, and in the 

 Isle of Wight — we have few outcropping rocks 

 in south-eastern England ; the chief examples of 

 inland rocks being those formed by the massive 

 beds of the Tunbridge Wells sandstone, such as 

 those near Tunbridge Wells and West Hoathly. 

 These sandstone rocks are the habitat of the filmy 

 fern [Hymenophylliim tujibridgense), now, I fear, extinct 

 at the locality from which it takes its name, but 

 still existing at a few secluded spots in the Weald. 



The harder strata are all commonly permeable to 

 water, and thus, if above the line of saturation, 

 form thin dry soils, suitable for growth of certain 

 plants often of a dwarf habit. In tracts formed 

 wholly of permeable strata, as the chalk plateau of 

 Surrey, aquatic plants and animals are necessarily 

 rare and few in species, being met with only in 



c 



isolated artificial ponds, where they have been 

 introduced by some casual agency. It is not 

 infrequent to find isolated ponds tenanted by a 

 single species of water-snail, while in waters con- 

 nected with an extensive waterway, a variety of 

 species are commonly present. 



Moist places occur where water lodges in hollows 

 on impervious soils and where springs break out at 

 the junction of pervious and impermeable beds. The 

 greatest variety of aquatic and marsh plants is 

 commonly found in low-lying tracts near the lower 

 reaches of large rivers, the dissemination of such 

 plants being assisted by floods. 



Impermeable strata, especially clay, form a 

 heavy soil which supports a strong coarse vegeta- 

 tion, consisting largely of perennials and trees, or 

 shrubs ; while on the other hand, dry, hard and 

 permeable beds are covered commonly by a thin 

 soil and a short velvety turf. 



The difference in the flora growing on subsoils of 

 different chemical composition is connected, no 

 doubt, partly with differences in their physical 

 characters, but also partly depends upon differences 

 in their chemical constituents, e.g., the presence or 

 absence of lime. Calcareous strata, of which in the 

 south-east of England the chalk is our main ex- 

 ample, are typically dry, hard and permeable. 

 Where superficial, as on the slopes of the chalk 

 downs — the plateaux being often capped with 

 gravelly or clayey drift — a calcareous rock is 

 covered with a thin loamy surface soil and a dense 

 green velvety turf. The flora of a calcareous tract 

 is notable for its great variety, the number of species 

 met with on such a tract being much larger than 

 tha,t on one of sand, clay or peat. It also com- 

 prises many typical species rarely or never met with 

 on other than calcareous soils. Among such species 

 may be mentioned rockrose, hairy violet, wild mig- 

 nonette, lesser burnet (Poterium), dropwort {Spiraea 

 filipendula), wild parsnip and carrot, marjoram, 

 juniper, yew, and bee orchis, while the list might 

 be greatly extended. There is some difference 

 between the flora of the harder and drier beds, 

 such as the Upper Chalk, and the heavier marls, 

 such as the Lower Chalk. Some species, such as 

 Spiraea filipendula and Hippocrepis comosa, preferring 

 the former, while others, such as Chlora perfoliata, 

 and various species of orchids, prefer the latter. 

 On the other hand, a calcareous soil is equally 

 characterised by the absence of plants, common 

 and abundant on other soils, such as foxglove, 

 heaths, bilberry, and some of the larger ferns. 



Chalk downs and hills on a calcareous soil are 

 the special habitats of many species of insects, 

 such as burnet moths {Zygaena filipendnlae), marbled- 

 white butterflies, and several of the blue butterflies 

 [Lycaena). A calcareous soil, too, is specially 

 favourable to the abundance of land molluscs, 

 doubtless on account of the plentiful supply of 



