SUMMARY AND CONCLUSION 295 
ited by insects, and thus of insuring cross-fertilization, 
which appears to be distinctly advantageous to the 
plant. From these probably accidental variations have 
been developed the mechanical devices for insuring 
cross-fertilization, as well as infinite varieties of color 
anid form, and the production of nectar and odors, serv- 
ing as lures to attract insects. The extraordinary 
development of the Angiosperms and Insects, the two 
largest divisions of the vegetable and animal kingdoms 
respectively, is to a very great degree correlated, the 
two groups being largely dependent upon each other 
for their existence. 
While provision for the development of seed is one 
of the most important functions of the plant, their dis- 
tribution is also necessary, and many arrangements for 
this have been evolved. The development of edible 
seeds and fruits, and of the numerous organs like the 
wings of such fruits as those of the maple and ash, or 
the down in the thistle or milkweed, the hooks and 
prickles upon the fruits of many Composite and Bor- 
raginew, are all devices for facilitating the distribution 
_of the seeds through the agency of the wind or by 
animals. 
Extensive modifications have arisen in the plant by 
which it adapts itself to a changed environment or pro- 
tects itself against the attacks of animal enemies. The 
earliest plants were probably aquatic, and their de- 
scendants, but little changed, still exist in the low 
green alge. The change from fresh to salt water has 
no doubt changed the marine forms profoundly, this 
being especially marked in the red and the brown alge, 
which differ widely from their probably more primitive 
