134 
POPULAE SCIENCE EEVIEW. 
Making every allowance, then, for the influence of soil and 
climate in checking the multiplication of individuals, we have still 
two classes of facts to account for ; the one, that plants which 
succeed so well, when cultivated, that we are assured both soil and 
•climate are favourable to their propagation, nevertheless become 
immediately or soon extinct when the cultivator’s care is with- 
drawn ; the other, that plants of one country, when introduced 
into another, even with a very different soil and climate, will 
overrun it, destroy the native vegetation, and prove themselves 
better suited to local circumstances than the aboriginal plants of 
the country. In the first case, the reasons are very various, 
all of them relating to the conditions of the plant’s existence. 
Of these the two most potent are, the absence of fertilising 
agents, and the destruction of seeds and seedling plants. In 
the present state of our knowledge it is impossible to say which 
of these is most fatal in its effect. In the case of our annual 
plants, or our cereals, which never run wild, it is the latter 
certainly, for they seed freely enough; in the case of many 
perennials, shrubs, and trees, it may be the former, as with the 
common elm and lime, which rarely or never seed in England, 
though the latter is so notably frequented by insects during its 
flowering season ; whilst a third cause is to be found in their 
seedling plants being smothered by others, of which we have 
numerous examples in our common pasture grasses, which are, 
perhaps, the most prejudicial in this respect. A most conspicuous 
example of this is afforded by the common maple, of which the 
seedlings come up early in spring by thousands in the neighbour- 
hood of the parent tree, in lawns and plantations, but scarcely 
ever survive the smothering effects of the common summer 
grasses, as soon as these begin to shoot. 
When I visited the cedar grove on Mount Lebanon, in the 
autumn of 1860, 1 found thousands of seedling plants, but every 
one of them dead ; and so effectual is the annual slaughter of 
the yearlings in that grove, that, though the seeds are shed in 
millions, and innumerable seedlings annually spring up, there is 
not a plant in the grove less than about sixty years old. It 
may hence have been sixty years since a cedar there survived the 
first year of its existence ; that is to say, has struggled through 
its infancy, and reached the age even of childhood ! 
On the other hand, when once the natural conditions of a 
country have been disturbed, the spread and multiplication of 
immigrants is so rapid, that it shortly becomes impossible to 
discover the limits of the old, indigenous Flora. Take the 
English Flora, for example. If we contrast the cultivated counties 
with the uncultivated, the difference of their vegetation is so 
great, that I have often been compelled to doubt whether many 
of the most familiar so-called wild flowers of the cultivated 
