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impulses, the animal adopts a new line of action, the law 
by which the use of a limb strengthens that limb, and the 
purposive movement tends to become automatic, must be 
of very great value, as those will admit who have learned 
to appreciate the difference of facility between performing 
an accustomed action and one which has never been 
attempted before. 
There can be no doubt that there is a correlation 
between the outward conditions of deserts and the vege 
table organisms which flourish more or less in such localities. 
In an arid district, overrun with cattle and rodents, spin- 
escent leaves would act as a protection from attack, and 
would repeat the warning of the thistle—Noli me tangere. 
And it is equally clear that any formation of growth which 
tended to enable the plant to resist heat and long drought, 
would be undoubtedly useful. Where plants remain essen 
tially unchanged in such conditions, it is because they 
have become annuals, and flourish during the rainy season 
when the problem of resisting the combined effects of heat 
and drought does not arise. In some cases a remarkable 
provision is made to preserve the race. Bulbs of species of 
Allium store water within the inner scales, whilst the 
outermost series become woody in texture as a protection 
against the hot sand in which they lie, the temperature of 
which sometimes rises to 130° Fah. There are plants in 
which certain buds swell into tubercles, capable of enduring 
the dry season, while the rest of the plant perishes. 
Where desert plants remain unmodified in the structure 
of the leaf, it is because they have secured an abundant 
supply of water. The long roots of certain plants reach a 
supply of water which would be otherwise unattainable. 
The Colocinth has an enormous length of root. It stands 
singly, it has large herbaceous leaves, it is without any means 
of preventing an excess of transpiration, for a cut shoot