REPRODUCTIVE ORGANS OF PLANTS. 29? 
Species, therefore, cannot change any of their essential 
characters, those characters which are hence termed specific. 
Varieties.—Individuals of the same spevies differ. In 
fact, no two individuals are quite alike. Circumstances of 
temperature, food, and habits of life, increase these differ- 
ences, and varieties originate when such differences assume 
a comparative permanence and fixity. But as external 
conditions cause variation away from any particular rep- 
resentative of a species, so they may cause variation back 
again to the original, and although variation may take a 
seemingly wide range, its bounds are fixed and do not 
touch specific characters. 
The causes that produce varieties are numerous, but in 
many cases their nature and their mode of action is diffi- 
cult or impossible to understand. The influence of scarcity 
or abundance of nutriment we can easily comprehend may 
dwarf a plant or lead to the production of a giant indi 
vidual; but how, in some cases, the peculiarities thus im- 
pressed upon individuals acquire permanence and are 
transmitted to subsequent generations, while in others 
they disappear, is beyond explanation. 
Among plants, varieties may often be perpetuated by 
the seed. This is true of our cereal and leguminous 
plants, which reproduce their kind with striking regulari- 
ty. Other plants cannot be or are not reproduced unalter- 
ed by the seed, but are continued in the possession of their 
peculiarities by cuttings, layers, and grafts. Here the in- 
dividual plant is in a sense divided and multiplied. The 
species is propagated, but not reproduced. The fact that 
the seeds of a potato, a grape, an apple, or pear, cannot be 
depended upon to reproduce the variety, may perhaps be 
more commonly due to unavoidable contact of pollen 
from other varieties, than to inability of the mother plant. 
to perpetuate its peculiarities. That such inability often 
exists, is, however, well established, and is, in general, 
most obvious in case of varieties that have to the greatest 
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