144 ORIGIN OF PLANT VARIATIONS 



some being absolutely necessary, others having only slight efifect 

 upon it. The character is merely the end product of a series of 

 reactions. Therefore you are not to think of these characters 

 as definite chemical substances that are all lodged in the indi- 

 vidual at the start of its life and that maintain their identity 

 unchanged throughout its development. At the start of an 

 organism a definite number of factors are involved — just enough 

 to give direction to its growth and to cause it to follow a 

 pattern of development that is peculiar to its kind. As growth 

 goes on owing to the interaction of these factors and also owing to 

 the action of the environment characters are constantly created 

 and here and there in the organism characters appear that give 

 us ocular evidence of the presence of the factors which are the 

 deciding cause of a color, of a form, of a size, etc. 



It will be seen from the above discussion that no cross can 

 result in a new creation. All the work in breeding shows that 

 the crosses result merely in new combinations of the old factors 

 that already existed in the parent stock. How then does vari- 

 ation come about and new characters arise? de Vries has 

 shown that one or more new characters may suddenly appear in 

 the offspring through the influence of unknown causes. These 

 sudden changes are termed mutations. It appears reasonable 

 that the gradual accumulations of these mutations going on 

 over a long period of time may have been the principal cause of 

 the enormous number of variations that are seen in plant and 

 animal life. 



What then is the purpose of fertilization? We see that it is a 

 process for effecting crossing and so bringing about a new combi- 

 nation of the characters and the variations that appear in the 

 parent plants. Some of these new combinations will be of no 

 value, others will give their possessors an advantage because 

 they adapt them to the conditions under which they live. 

 These latter forms will therefore tend to survive and to crowd 

 out the less favored. This is natural selection, the environment 

 bringing about the survival of the fit. As long as the environ- 

 ment remains constant and natural selection has picked out forms 



