g^2 ORIGIN OF SPECIES. 



How complicated may be this relationship in the cases of only two nearly related 

 forms of plants in their struggle for existence in particular localities has been described 

 as exhaustively as clearly by Nageli in the case of various Alpine plants. *Thc interne- 

 cine war,* he says \ * is obviously most severe between the species and races that are 

 most nearly related, because they require the same conditions of existence. Achillea 

 moschata drives out A, atrata, or is driven out by it ; they are seldom found side by side ; 

 while each grows along with A. Millefolium. It is clear that Achillea moschata and atrata^ 

 being extremely similar to one another externally, make similar demands on their en- 

 vironment, while A. Millefolium, which is less nearly allied to both, does not properly 

 compete with them, because it requires other conditions of existence. Still less do 



plants of different genera or orders compete with one another In the Bernina- 



Heuthal (Upper Engadin) Achillea moschata^ atrata, and Millefolium occur in profusion, 

 A. moschata and Millefolium on slate, A. atrata and Millefolium on limestone ; where 

 the slate ends and limestone begins, A. moschata always ceases and A, atrata takes its 

 place. Both species are therefore here strictly circumscribed as to soil, and this I 

 have found to be the case also at various spots in Biindten, where both species occur 

 together. But where one species is absent the other is widely distributed. A. atrata is 

 then found indiscriminately on slate or limestone : and although A, moschata does not 

 apparently grow so readily on limestone as A. atrata does on slate, yet in the neighbour- 

 hood of the primary rocks it is found on a distinctly calcareous formation along with the 

 vegetation characteristic of it. In the Bernina-Heuthal I found in the midst of the slate 

 which was thickly covered with A. moschata a large erratic block of limestone covered 

 with a crust of soil scarcely an inch thick, upon which a patch of A. moschata had 

 established itself, because it did not here meet with any competition from A. atrata. . . . 

 A similar relationship was observed in certain districts between Rhododendron hirsutum 

 and ferrugineum, Saussurea alpina and discolor, and between species of the genera Genti- 

 ana, Veronica, Erigeron, Hieracium, &c.' The obvious objection, that there cannot 

 possibly be any struggle between two forms of plants as long as there is space for 

 both in the area in question, rests on an incorrect basis, and is disposed of by Nageli 

 as follows : — * Upon a slate slope are a million plants of A. moschata ; they obviously do 

 not occupy the whole space, for a hundred millions or more could find room there ; but 

 the rest of the space is occupied by other plants. There is here a condition of equi- 

 librium, which has been produced in reference to the nature of the soil and the preceding 

 climatic influences. The number one million gives us also the proportion which A. 

 moschata is able to maintain in relation to the rest of the vegetation ; and the objection 

 that there would still be plenty of room for A. atrata is an untenable one. If the space 

 were accessible to species of Achillea generally, it would be occupied by the species which 

 is already present, and which in any case has the advantage, A. moschata. If we now 

 imagine that the two species happened for once to be intermixed on the slate slope, 

 perhaps in consequence of artificial transplanting, in equal quantities, say 500,000 plants 

 of each, A, moschata would thrive the better of the two, as the soil contains but little 

 lime; A. atrata would become weaker and its tissue less matured, and would in conse- 

 quence have less power to withstand external prejudicial influences, as summer frosts, 

 long-continued rainy weather, or persistent drought, &c. If we suppose, for example, 

 that every twentieth or fiftieth year a severe frost occurs at the time of flowering 

 which destroys half the plants of A. atrata, while the more vigorous A. moschata re- 

 sists it, the voids are again filled up by the dispersion of the seeds ; but more plants of 

 A. moschata spring up than of A. atrata, because the number of individuals of the latter 

 was reduced by the frost to 250,000, while that of the former remains at 500,000. The 

 million plants of Achillea on the slope will in consequence be composed of say 670,000 



^ Sitzungsber, der kon. bayer. Akad, der Wiss. Dec. 15, 1865. The manner in which the arrange- 

 ments for the protection of the pollen with reference to certain insects on the one hand, and to 

 climatic influences on the other, determine the distribution of certain plants, is clearly indicated 

 by Kerner (Schutzmittel des Pollens, Innsbruck 1873). 



