THE TIGRIDIA 161 



And this method of propagation is a perpetual 

 bid for such variation as will make possible a 

 relatively rapid change in adaptation to a chang- 

 ing environment. 



That vast tribes of plants should have found 

 it necessary to adopt both methods of propaga- 

 tion is in itself an evidence of the struggle for 

 existence that is the basis of natural selection. 



In another way, also, the bulb perhaps evi- 

 dences the hardness of the struggle for existence, 

 particularly in tropical climates. Everyone 

 knows that vegetation is exceedingly luxuriant 

 in the tropics, and it is a matter of observation 

 that the habit of developing tubers and bulbs is 

 especially conmion among the herbaceous plants 

 of tropical and subtropical regions. Perhaps 

 one explanation is that the storing of food sup- 

 plies in the bulb enables the young plants to 

 shoot up rapidly without waiting for the develop- 

 ment of a large root system. 



By so doing they may stand a chance of com- 

 peting with the surrounding vegetation and thus 

 have a far better chance of reaching maturity 

 than if they had grown from tiny seeds. 



It is probable, therefore, that the generality of 

 bulbous plants that one would find in any given 

 locality in their native haunts would have devel- 

 oped as offshoots of the bulb of an original plant 



Vol. 7 — Bur. F 



