DEFENSIVE EQUIPMENT OF PLANTS _ 139 
be seen dwarfed and cropped close to the ground, where 
rabbits abound ; others, doubtless, are entirely extermi- 
nated through the ravages of the resident animals. 
The structures found on plants which serve as a means 
of defence against living enemies were not actually called 
into existence for that purpose. The leaves of the holly, 
for example, are not tough and spiny to protect the plant 
against the hungry mouths of animals, although, being 
prickly, they do as a matter of fact serve that purpose. 
Nor does the gorse grow its thorns and the bramble its 
prickles to make their bodies disagreeable eating. True, 
the thorn and the prickle do secure their owners from 
injury where browsing animals abound, and in the course 
of time plants not so protected might in such localities 
become exterminated. But the animal is not the cause 
of the thorn. Thorny plants would grow even if no 
browsing animals existed on the earth. They are char- 
acteristic plants in dry, hot regions where extreme xero- 
phytic conditions prevail. In a xerophyte the transpira- 
tion-current is poor and slow. The plant is consequently 
badly nourished, and the spiny or prickly habit is simply 
the result of this defective nutrition. In xerophytic 
regions vegetation is scanty, and the animals are not too 
well supplied with food. The few plants that are found 
there would consequently stand a poor chance of surviving 
if they did not possess some means of keeping off their 
destroyers. But the very evils from which they suffer 
provide a means of safety. Dearth of water means lack 
of food. Lack of nourishment sets up all kinds of physio- 
logical disturbances. It affects assimilation, transpira- 
tion, respiration, and growth. Asa result, leaves become 
tough and indigestible ; buds become provided with scales, 
and form gummy excretions ; spines, thorns, and prickles 
develop at the expense of assimilating tissue, while gums, 
waxes, oils, and bitter and poisonous juices form and 
collect in the organs. Once being produced, and being 
found at the same time useful, these things have been 
preserved by Natural Selection, and a specialization has 
been marked out for them upon the lines of protection 
and defence. 
