272 



EVOLUTION AND GENETICS 



Influence of the Discovery of Cells. Although the celhi- 

 lar structure of organisms was recognized long before any impor- 

 tant contributions to the study of heredity appeared, detailed 

 knowledge of cell structure and behaviour was not available for 

 many years. Mendel's important discoveries described in the 

 next chapter were made without such knowledge, but they have 

 much greater significance with the background of modern cytology 

 than when they stand alone. The importance of cell behaviour 



Fig. 156. — Diagram to illustrate Galton's law of filial regression. The circles 

 represent graded parental heights while the arrow points indicate the 

 average height of offspring descended from each group. The offspring of 

 short parents are taller and of tall parents shorter than their respective 

 parents. (From Walter.) 



is strongly suggested by the methods of reproduction in organisms 

 of various degrees of development. 



Reproduction of Unicellular Organisms. Single-celled plants 

 and animals reproduce in many cases by an equal subdivision 

 which gives rise to two new individuals with complete loss of 

 individuality on the part of the parent (Fig. 157). Such offspring 

 are apparently as nearly as possible identical with each other 

 and with the parent; there is abundant reason to believe them at 

 least potentially the same. 



Occasionally these single-celled forms undergo a complex process 



