CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. 833 



primitive form, to a less extent between different species and genera. The result of 

 these relationships is seen on the one hand in the fact that with plants which live 

 socially only the most vigorous seedlings arrive at full maturity, while the weaker ones 

 are smothered, as may be seen in any young plantation ; on the other hand, that 

 species and genera which differ greatly from one another can thrive side by side, 

 because their requirements are different and the competition between them is less. 



From the fact that plants whose organisation differs can thrive better side by 

 side on the same soil in consequence of the diminished competition between them, 

 Darwin drew the important and pregnant conclusion that in the propagation of the 

 varieties of one primitive form those new forms must be the best able to maintain 

 themselves in the wild state which differ most from the primitive form and from one 

 another, while the intermediate forms are gradually dispossessed. This is the reason 

 why the connecting forms between the different species of a genus are so often want- 

 ing, although the conclusion cannot be avoided that the species arose by variation 

 from a single ancestral form, and by the propagation of varieties. 



In its larger features (but on that account more conspicuously) the struggle for ex- 

 istence between the various forms of plants, the competition for space, food, and light, 

 are manifested in the luxuriant growth of what we term weeds in our gardens and fields. 

 Our cultivated plants are able to bear our climate, and the soil supplies what they 

 require for their vigorous growth. But a number of wild plants are still better adapted to 

 the climate ; and they grow still more vigorously, rapidly, and luxuriantly on cultivated 

 soil, and their seeds or rhizomes are everywhere present in enormous quantities. If 

 the cultivated plants are not carefully protected from the weeds, the latter soon dis- 

 possess them of the ground which was set apart for them. Every country and every soil 

 has its own peculiar weeds ; i.e. under any particular external conditions there are always 

 certain forms of plants which thrive best and drive out the cultivated plants. To a 

 certain extent we have a measure of the amount of advantage which weeds have over 

 cultivated plants in the amount of labour bestowed by man on their destruction in 

 order to preserve and maintain his nurselings. The primitive forms of our cultivated 

 plants are mostly natives of other countries, where they are not only sufficiently adapted 

 for the climate, but are able to sustain competition with their neighbours. 



The number of species or of individuals of any species which we find in a meadow, 

 a marsh, &c. is not a matter of chance ; it does not depend merely on the number of seeds 

 of one or another species produced or brought to the locality ; every one of these spe:ics 

 would, if it alone existed there or were protected by cultivation, of itself cover the space 

 of ground in a short time ; and yet there is a definite relationship between the numbers 

 of individuals of the different species when left to themselves, a relationship which de- 

 pends on the specific power of each particular species to maintain itself in the struggle 

 with the rest^. 



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 Niigeli in the case of various Alpine plants. ' The interne- 

 cine war,' he says^, Ms obviously most severe between the species and races that are 

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



* [How the relationship subsisting between the species in permanent pastures may be disturbed 

 by the application of different manures, may be seen in Lawes and Gilbert's paper on this subject 

 in Journ. Roy. Agric. Soc. vol. XXIV, 1863.— Ed.] 



2 Sitzungsber. der Icon, bayer. Akad. der Wiss. Dec. 15, 1865. 



3 H 



