832 ORIGIN OF SPECIES. 



although usually to an imperceptible amount. On the other hand, the struggle for 

 existence in plants (as well as in animals) is a perpetual and never-ceasing one, in 

 which the smallest advantage that the plant has obtained through variation in any 

 one direction may be of the utmost importance for its perpetuation. 



The struggle which the plant carries on by means of its capacity for variation 

 has two different aspects. On the one hand its tendency is to adapt the organisation 

 of the plant completely to the conditions of food and growth afforded by the climate 

 and the soil. It is evident that the organisation of a submerged water-plant must be 

 different from that of a land-plant ; that the assimilating organs of a plant that grows 

 in the deep shade of a wood must be differently constructed from those of a plant 

 exposed daily to bright sunshine, and so forth. The vital conditions of all plants 

 growing at a great elevation and in Arctic countries must be different from those 

 growing in the lowlands of the Tropic and Temperate zones. If we had to do only 

 with the general conditions of plant-life, the struggle for existence would be a com- 

 paratively simple process. It would be easy to imagine how, among the varieties of a 

 primitive form which grew in water, there would be some which would be occasionally 

 subjected to a subsidence of the water, and how these would give birth to descend- 

 ants which would gradually assume the character of marsh- and finally of land-plants, 

 as is well illustrated in the case of Nasturtium amphibium^ Polygonum amphibiuniy 

 &c.^ It may also be supposed that some of the descendants of a plant exhibit a 

 somewhat greater power of resisting frost, that this property increases in the course 

 of generations, and that thus a form which can at first only bear a temperate climate 

 gradually produces varieties which can endure a more and more severe climate ; and 

 so forth. But these comparatively simple relationships must lead to a great diversity 

 in the varieties which claim descent from one ancestral form ; for each adaptation to 

 new conditions of climate or locality would act in different ways ; z*. e. varieties of 

 different descriptions would take up and carry out in different ways the struggle 

 against the influences of the elements. 



But the struggle for existence and the changes occasioned by it in the organ- 

 isation of plants are greatly complicated by the fact that every plant, while struggling 

 to adapt itself to its special vital conditions, has also to protect itself at the same time 

 against a number of other plants and against the attacks of animals ; or, what is more 

 to the point, its capacity for variation enables it to make use of particular favourable 

 conditions which are offered to it by other plants and animals in order to take ad- 

 vantage of them ; as parasites of their hosts, dichogamous and other flowering plants 

 of the visits of insects, &c. These relationships are endless in their diversity, and 

 can only be illustrated by examples. 



But we must here call special attention to a remark of Darwin's ; that the indi- 

 viduals of the same species or variety are competitors for position, food, light, &c. 

 The fact that plants of the same species have the same requirements itself gives rise 

 to a struggle for existence among them ; and the same is the case, though to a some- 

 what smaller but still to a great extent between the different varieties of the same 



^ A special interest attaches in this connection to Hildebrand's observations on Marsilea in Bot. 

 Zeit. 1870, No. I, and Askenasy's on Ranunmlus aquatilis and divamatus in Bot. Zeit. 1870, p. 193 

 et seq. 



