ANIMALS AND PLANTS 245 
less. In such plants a large amount of pollen is neces- 
sary in order that fertilization may be insured, as a very 
large part of it fails to reach the carpels. An extreme 
case of this is seen in the pines and firs, where the 
amount of pollen is enormously in excess of what is 
actually needed for fertilization. 
The development of contrivances by which the trans- 
fer of pollen to the pistil is facilitated, results in an 
obvious saving of pollen, and is in itself an advantage ; 
but experiment has demonstrated that cross-fertilization, 
ze. pollination of one flower by pollen from another 
one, is generally of advantage to the plant, as seed so 
produced is usually more vigorous than when the 
ovule is fertilized by pollen developed in the same 
flower. 
The simpler flowers have no enveloping leaves, and 
the first step toward the development of a floral en- 
velope or perianth was probably the production of 
small scale-like leaves, either green or membranaceous 
in texture. The change from these inconspicuous, 
purely protective floral leaves, to those which are more 
or less conspicuously colored, marks the next advance 
in the evolution of the flower. This bright-colored 
corolla would no doubt make the flower more conspic- 
uous, and attract insects in search of pollen. Such an 
insect visiting the flower would be pretty sure to carry 
some of the pollen to other flowers of the same kind, 
insuring cross-fertilization. As a result of natural se- 
lection, it is easy to conceive how flowers having the 
showiest corollas would stand a better chance of at- 
tracting insects and thus being cross-fertilized. These 
plants would produce a greater amount of seed, and in 
