INTRODUCTION 57 



comprehend that in each different set of circum- 

 stances the individual will respond differently, and 

 that this response will have an effect upon the 

 structure or form of the individual. As these 

 modifications accumulate the plant will differ con- 

 siderably from the original type. The response of 

 the internal factors to the external factors is a 

 measure of the amount of variability. 



The organs of the plant will thus become altered 

 as they have to perform different functions. Even if 

 the conditions are more or less the same the indi- 

 viduals will tend to vary, owing to the intense 

 competition. Some will die, others will survive. 

 The plants that exhibit some character which is of 

 advantage to them in such conditions will persist or 

 survive, the rest will succumb. Such a character by 

 natural selection will then be developed, and the 

 variation or adaptation which is of use will be 

 transmitted to succeeding generations and become 

 fixed. Useless adaptations will be rejected. Variations 

 may be produced by artificial selection as in horti- 

 culture. 



When any character becomes fixed it is hereditary. 

 Heredity is opposed to variation by which the plant 

 tends to vary from its ancestral type. Moreover, 

 each individual of a species tends to preserve in the 

 main the characters of the parent, and this is due to 

 the influence of heredity. Variation may be direct 

 when plants of different type are crossed ; when the 

 variation is due to the influence of the environment 



