CLIMATE AS A BARRIER. 881 



where the soil is favourable, but where the intensity of light, of warmth, or of 

 moisture exceeds or falls short of the right measure for that particular species, the 

 development of the species is arrested at the outset, and the plants die without 

 leaving any oflFspring behind them. In this manner an absolute barrier is opposed 

 by climatic conditions to the dispersion of each species. It must be added that the 

 check may be given in one direction by one factor and in another direction by another 

 climatic factor, and that not infrequently many conditions, collectively classed under 

 the name of climate, exercise a simultaneous influence on the distribution of species. 



The limits to the range of plants towards the Arctic and Antarctic regions and 

 towards the summits of high mountains are imposed by the diminution of tempera- 

 ture and the increasing length of the winter, whilst the opposite boundary is 

 encountered where the duration of daylight is still too short at the time of year 

 when the temperature begins to be sufficient to cause the plants in question to 

 sprout. The continental climate, which is distinguished by slight degrees of mois- 

 ture, high summer temperatures, and low winter temperatures, checks those plants 

 which suffer from dryness in summer or which cannot endure the cold of winter. 

 On the other hand, in the case of species whose transpiration is unduly checked by 

 a high degree of atmospheric moisture and which require an elevated temperature 

 in summer in order to bring their seeds to maturity, bounds are set to dispersion by 

 the climate of the sea-coast where comparatively slight variations of temperature 

 occur during the year and where the summers are cool and the air damp. Meteoro- 

 logists show us on special charts the distribution of the climatic factors by connecting 

 all places having the same mean winter temperature, the same mean summer tem- 

 perature, the same mean annual deposition of moisture from the atmosphere, and so 

 forth, by lines which are termed isocheimal, isotheral, and lines of like mean annual 

 rainfall respectively. The distribution of plants, in so far as it depends on climatic 

 conditions, may be shown in the same manner by drawing lines connecting all the 

 places at which any species is checked by climatic conditions. Such lines are called 

 lines of vegetation, and when they run along the slopes of a mountain they coincide 

 with the contour-lines. As each species of plant is checked in its progress towards 

 the different quarters of the compass by different factors of climate, lines of vegetation 

 may be drawn corresponding to the limits of range for each species to the north, 

 north-east, east, south-east, south, &c. When all these vegetation-lines of a species 

 are connected we obtain a curve which returns upon itself and is called a line of 

 distribution. In most cases this line resembles an ellipse with the longer axis lying 

 in the direction of the parallels of latitude. It is, however, not infrequently modi- 

 fied by influence of the nearest lines of sea-coast. The proximity of mountains also 

 may cause variations which are principally of the nature of sinuses or bulgings. 



The line of distribution incloses therefore the entire area of distribution in which 

 the species in question finds suitable conditions and in which as a fact it grows and 

 multiplies. Emphasis must be laid on the latter circumstance, because experience 

 has shown that a plant-species does not necessarily grow in all the places where the 

 conditions are favourable to its existence. Only the boundary-lines of the area of 



VOL. II. ^"^ 



