ee 
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J. D, Hooker, Introductory Essay to the Flora of Tasmania. 15 
annual destruction falls upon the eggs or seeds and young of the 
organisms, and as it is effected by a multitude of antagonistic, 
ever-changing natural causes, each more destructive of one or- 
ganism than of any other, it operates with different effect on 
each group of individuals, in every locality, and at every re- 
turning season. Here then we have an infinite number of vary- 
ing conditions, and a superabundant supply of variable organ- 
isms, to accommodate themselves to these conditions. the 
organisms can have no power of surviving any change in these 
conditions, except they are endowed with the means of accom- 
modating themselves to it. The exercise of this power may be 
accompanied by a visible (morphological) change in the form or 
structure of the individual, or it may not, in which case there is 
still a change, but a physiological one, not outwardly mani- 
fested; but there is always a morphological change if the change 
of conditions be sudden, or when, through lapse of time, it be- 
comes extreme, e new form is necessarily that best suited to 
the changed condition, and as its progeny are henceforth addi- 
tional enemies to the old, they will eventually tend to replace 
their parent form in the same locality. Further, a greater pro- 
portion of the seeds and young of the old will annually be de- 
stroyed than of the new, and the survivors of the old, being 
less well adapted to the locality, will yield less seed, and hence 
have fewer descendants. 
In the above operations Nature acts slowly on all organisms, 
but man does so rapidly on the few he cultivates or domesticates ; 
he selects an organism suited to his own locality, and by so modi- 
fying its surrounding conditions that the food and space that 
were the share of others falls to it, he ensures a perpetuation of 
his variety, and a multiplication of its individuals, by means 
the destruction of the previous inhabitants of the same locality ; 
and in every instance, where he has worked long enough, he 
finds that changes of form have resulted far greater than would 
suffice to constitute conventional species amongst organisms in a 
state of nature, and he keeps them distinct by maintaining these 
conditions. 7 
isolated areas the number of classes, orders, and genera is very 
large in proportion to that of species. 
$3. On the General Phenomena of Distribution in Area. 
_Turning now to another class of facts, those that refer to the 
distribution of plants on the surface of the globe, the following 
are the most obvious :— 
