loo METABOLISM 



the Heu-Thal ; atrata on chalk and moschata on slate. ' Wherever the slate passes 

 into limestone, moschata at once stops and atrata begins.' In other places, 

 however, where only one species occurs, it appears quite indifferent as to whether 

 the substratum be slate or limestone. So long as only one is present both 

 plants are indifferent as to the soil, but when both occur in company they at 

 once become confined to special areas ; they adapt themselves markedly to 

 the chemical nature of the soil and are indifferent to the physical conditions, 

 for they thrive equally well in wet places and dry places, on humus, on sand, or on 

 rock. The mutual exclusiveness of these species oi Achillea can be explained only 

 by assuming the influence of concurrence or of the struggle for existence between 

 nearly related species as the effective cause. Each species lives only on a soil 

 where the conditions are better fitted for its existence. Nageli's experiments 

 do not demonstrate wherein lies the reason for Achillea atrata' s preference for 

 limestone and moschata's preference for slate. It may be concluded, however, 

 that this preference is quite limited, and yet it may be sufficient to determine, 

 in a state of nature, the existence or non-existence of the species. We are also 

 familiar in our own country with plenty of examples of rapidly spreading 

 American weeds which are capable of killing our native plants, and even entire 

 floras, in a very short time ; but we are quite ignorant as to wherein lies their 

 power of intruding and ousting out the native flora. A glance at our culti- 

 vated plants is sufficient to prove to us that organisms, when withdrawn from the 

 company of others, are able to exist under conditions which they cannot 

 tolerate in the wild state. Although we are unacquainted with the reasons 

 why Achillea atrata gains the ascendancy in one case and Achillea moschata in 

 another, still we must accept the fact that association is a factor of the highest 

 importance in determining the distribution of plants on the earth's surface. 



Nageli has drawn attention to another important factor to account for 

 the irregular occurrence of plants which we have space only to glance at in 

 passing. A plant may be absent from an area, notwithstanding the fact that 

 the chemical and physical conditions of the soil, the plant society, and the 

 general climatic features are favourable, simply because none of its seeds have 

 as yet been distributed to that district (Historical Plant Geography). 

 1. We must content ourselves here with this brief outline ; further details 

 will be found in the works of Schimper (1898) and Engler (1879). One point 

 at least is clear, viz. that these problems of plant geography are exceedingly 

 complicated and cannot be settled off-hand. In fact, the unfortunate craze 

 for looking for one cause instead of several only tends to obscure the real issue. 

 When, in the future, accurate researches on this question come to be pieced 

 together, other factors than those we have alluded to will, doubtless, be dis- 

 covered. One result, however, these researches certainly will have, they will 

 tend to discourage the forming of summary conclusions and draw more pointed 

 attention to the details in individual cases both as regards the plant and also 

 the soil, and so, doubtless, lead to the discovery of far more individual and 

 specific differences than could be expected from the older researches on the 

 subject. 



In addition to salt, hme and siliceous soils, reference must in conclusion 

 be made to humus soils, which are characterized by supporting special plant 

 communities. We shall return to this subject later (Lecture XIX). 



Before we finally leave the consideration of the mineral constituents, we will 

 glance at cultivated plants as they are found in our fields and woods, and at the 

 same time gain a yet greater appreciation of the fundamental importance of 

 the minerals present in the soil. In nature, the covering of an area of soil by 

 vegetation leads, as we have seen, to its enrichment in nutritive substances, 

 since each plant on its death gives back to the soil what it took from it, and in 

 a form also which other plants can easily appropriate. It is true each material 



