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ORIGIN OF SPECIES. 



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 Bilndten, where both species occur 

 together. But where one species is absent the other is widely distributed, and is then 

 found indiscriminately on slate or limestone. Although A. moschata does not apparently 

 grow so readily on limestone as A. atrata does on slate, yet in the neighbourhood 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 

 Sind 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 Niigeli 

 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 preced- 

 ing 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 

 hme; 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 

 A. moschata and 330,000 A. atrata. After a second frost, which again destroys one half 

 of the individuals of A. atrata, we should have about 800,000 of A. moschata to 200,000 

 of A. atrata. In this manner the number of the latter would decrease with every un- 

 usual summer frost, until at length it entirely disappeared, a nearly-allied hardier species 

 becoming distributed over the locality in its place.' In conclusion, the following remark 

 by the same author may be added : — ' From such a course of reasoning the conclusion 

 might perhaps be drawn that this result would always take place, and that one of two 

 plants would always be crowded out, because the two could hardly be precisely equally 

 hardy. But this conclusion would be unsound, because it would hold good only for 



