PLANT IN RELATION TO ITS BIOLOGICAL ENVIRONMENT. 20 9 



which penetrates deeply and ramifies widely. Each plant occupies a 

 fairly well-defined position, and its sphere of influence is perforce 

 respected. Each successful individual draws its water from the soil 

 within its own area, and unless an interloper, better adapted than the 

 original occupant, obtains a footing, it continues to exist alone, its 

 isolation representing a kind of biological balance of power. The very 

 fact of possession constitutes the title to its estate and to its own 

 place under the sun, and possession is ensured by a root system appro- 

 priate to the physical conditions of the situation. 



An analogous condition of things is also to be seen in the high Alps, 

 where the species are commonly sparsely strewn on cliff, moraine, or 

 mountain debris. This is partly to be attributed to the lack of avail- 

 able spots on a cliff, the shifting nature of the debris, and perhaps 

 more than all to the combination of the low temperature of the 

 soil, which restricts the rate of intake of water, with the drying 

 conditions that so often prevail above-ground. The result is to 

 produce many plants with remarkable root systems, though the pro- 

 blems are solved sometimes in other ways. Eritrichium nanum may 

 be instanced as a high alpine "desert " type. It grows to greatest 

 perfection, perhaps, on well-drained cliffs, and though it may be found 

 in other kinds of situation, I have never seen it so fine as high up on 

 the great south-facing precipice of the Meije, in Dauphine. Hundreds 

 of plants are growing there in a limited area, but they do not interfere 

 with each other. It is possible, by detaching a slab of rock and so 

 laying bare the roots, to understand the reason for their individual 

 isolation. The root system is enormous for so small a plant, and 

 that of a single individual may cover an area of several square feet 

 with a delicate network. When one has seen how Eritrichium grows 

 at its best, there is no room for surprise at its natural isolation, or 

 at the difficulty that attends its cultivation under the ordinary 

 conditions of the rock garden. 



But there are other factors which become operative amongst the 

 plants that are adapted to live under conditions apparently more 

 social. A crude competition is the simplest expression of such a state, 

 although as a matter of fact the situation is always seen to be more 

 complex under actual nature conditions. One commonly finds areas 

 occupied, not by a single species, but by many species, and these live 

 together in a nicely balanced proportion which is often maintained 

 over long periods of time. The physical conditions of moisture* 

 climate, soil, and the like, determine the gross outlines of the flora of 

 any particular locality, bat within those main limiting circumstances 

 the relations between plant and plant are adjusted with extreme 

 nicety, and may vary between sheer competition and absolute 

 interdependence. 



It is a significant fact, which hardly seems to have attracted the 

 full attention it seems to me to deserve, that one of the frequent 

 concomitants of specific differentiation consists in the lessening of the 

 direct competition between very closely allied forms. If one observes 



