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 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, 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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