[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 495 



due to the acceptance and retention of a too literal version of the fiction 

 with which we were entertained in childhood. The fantastic but enter- 

 taining Just So Stories in which Kipling tells us how the elephant got its 

 trunk and the leopard got its spots; or the equally fantastic stories in sup- 

 posedly more serious literature that tell us how the giraflFe got its long 

 neck and why certain reptiles have no legs, should be read and remem- 

 bered as interesting flights of fancy, but not allowed to interfere with 

 scientific thinking. They admirably illustrate the traditional belief in the 

 inheritance of acquired characters. 



If environment does change the heredity of a plant it must cause an 

 alteration in the composition or in the arrangement of the hereditary 

 units of matter. Furthermore, any change that is inherited through sexual 

 reproduction must occur either in the reproductive cells, or in cells that 

 are the forerunners of reproductive cells. 



Changes in the forerunners of reproductive cells which later become 

 inherited are much more likely to occur in plants than in animals be- 

 cause the reproductive tissue becomes differentiated from bodv tissue 

 much earlier in the development of an animal. Neither the reproductive 

 cells nor any forerunners of them are in the neck of the giraffe, in the 

 legs of a reptile, or in the trunk of an elephant. In a seed plant, however, 

 cells in the growing stem tips are remote forerunners of the reproductive 

 cells, which develop in the stamens and pistils of flowers after the plant 

 has passed through a vegetative phase of growth. Beginning with a 

 fertilized egg, or with a vegetative propagule, cells follow each other in 

 regular sequence by billions of cell divisions up to the immediate fore- 

 runners of the reproductive cells. Hereditary changes that occur in these 

 vegetative (body) cells previous to or during their division may be 

 transmitted by subsequent cell divisions to the reproductive cells. Even 

 this condition does not exemplify the traditional belief in the inheritance 

 of acquired characters, for that belief is based upon the assumption 

 that changes — including fluctuations — in mature body cells that are not 

 forerunners of reproductive cells may be transmitted to the offspring. 



The origin of new kinds of plants by evolution. There are two very 

 different methods of approaching a study of evolution. One method is 

 descriptive; the other is interpretative. The interpretative method will be 

 noted first. The fundamental processes in living organisms may be studied 

 until those that inevitably result in evolution are recognized. This is the 

 method of approach we have been using throughout most of this book. 



