114 MAN, — THE ANIMAL 



Study by reading Darwin's hypothesis of pan- 

 genesis. In the large treatises on heredity will be 

 found theories by Spencer, Altman, Weismann, 

 NaegeH, Galton, Roux, Driesch, Bateson, Men- 

 del and numerous writers of to-day, among whom 

 Castle, Morgan, Jennings, Davenport and Pearl 

 are the best known, and they should all be read 

 in connection with the question of how the 

 chromosomes transmit parental characters. 



In the illustrations accompanying this chapter, 

 it is to be noted that the complete transference 

 of parental characters to offspring does not 

 occur. There is always a distiguishable differ- 

 ence. We use the term variation in biology to 

 describe the differences of structure, of instincts 

 or of elements which occur between the offspring 

 and parent. 



If there were complete transference of heredi- 

 tary characters, then the offspring would be 

 identical with the parent and there would never 

 have been any progressive development in animals 

 and plants. 



The amount of variation that can take place 

 in a structure without changing the character of 

 the object is hardly appreciated until one makes 

 a critical study of the parts of living things. In 

 Fig. 39 is shown the range of variation in the 

 length and form of the heads of wheat. All of 



