AN OUTLINE OF PHYTOBIOLOGY. O 



In addition it secures to them the following advan- 

 tages, and these are of vast importance in the struggle 

 for existence. 



3. To give the new individuals opportunity to develop under 

 •environment somewhat different from that of the parents, thus 

 promoting vigor and variability, the two qualities upon which 

 progressive development in evolution depends. 



4. To mix individuals derived from different environments so 

 that cross-fertilization may occur between plants of different 

 stock, thus promoting in the offspring the advantageous vigor 

 and variability. 



5. To allow a race to move to new localities as it exhausts 

 certain essentials in the old. 



For the animal world should be added to these, to escape 

 from enemies, a condition which in plants must be otherwise 

 worked out. 



Among animals, all of these advantages are secured 

 to them by the power of independent movement which 

 they nearly all possess ; hut among plants, which lack 

 this power, the same end must be attained in a totally 

 diiferent way. One of the most fundamental diiferences 

 between animals and plants consists in this, that plants 

 being nourished by inorganic materials which are brought 

 to them by the movements of the atmosphere, or of water 

 in the soil, have not needed to move for the greatest of 

 all necessities, that of food ; their cells have accordingly 

 built up the lirm non-contractile cellulose and its deri- 

 vatives, a supporting skeleton capable of great size and 

 strength, but not of motion ; while animals, living upon 

 other organized beings, which they must go in search or 

 pursuit of, have formed a contractile substance, muscular 

 fibre, and the presence of this gives a muscular S3-stem 

 and the possibility of locomotion to even the largest ani- 

 mals. Yet that plants do in some way secure locomotion 



