CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. 93 1 



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. 



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 

 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, in the case of 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 §oil 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, whereas the intermediate forms must be gradually dispossessed. This is the 

 reason why the connecting forms between the different species of a genus are so 

 often wanting, 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 broader features (and on that account more conspicuously) the struggle for 

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

 is 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 species 

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

 ground in a short time ; but there is a definite relationship between the numbers of indi- 

 viduals of the different species when left to themselves, a relationship which depends on the 

 specific power of each particular species to maintain itself in the struggle with the rest^. 



^ [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.] 



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