HEREDITY 305 



Galton's Law. Many years ago Galton, studying by means of 

 statistics the inheritance of various traits, set down a law which 

 was widely discussed and is still occasionally quoted. In brief, 

 Galton's law holds that an individual inherits half its characters 

 from each parent, one-fourth from each grandparent, one-eighth 

 from each great grandparent, and so on back into its ancestry. The 

 law attempts to state in quantitative terms what in a general way 

 is the common observation of everyone, namely, that resemblances 

 between parent and offspring are closer than between grandparent 

 and grandchild, and that offspring resemble both parents. But it 

 tells us nothing of the mechanism of inheritance, and, as will be 

 apparent later, is definitely applicable only under conditions that 

 are rare indeed. 



The Cell Nucleus and Inheritance. Perhaps the most strik- 

 ing contribution of the present century to biological knowledge is 

 the success attained in working out the fundamental principles of 

 inheritance within the species. Obviously, the machinery of trans- 

 mission of characters from parent to offspring is in some way asso- 

 ciated with the reproductive cells. Spermatozoa and ova are very 

 different in appearance and in physiological nature, yet both carry 

 inheritable qualities. Ova consist essentially of a relatively large 

 quantity of cytoplasm, more or less heavily charged with yolk, and 

 contain a nucleus; spermatozoa have but a very small proportion of 

 cytoplasm and no yolk, the cell being largely nucleus. The nucleus 

 being the most distinctive feature common to both egg and sperm, 

 attention centres on it as the seat of the inheritable qualities. There- 

 fore in studying the nature and mechanism of inheritance, most 

 minute attention must be given the nucleus and its changes during 

 the passage from one cell generation to the next. 



Cell Division. No one knows just why a cell divides. In grow- 

 ing tissues there appears to be some critical relation between the 

 volume of the cell and the area of its surface, so that when a cer- 

 tain ratio of volume to surface area is reached, division into smaller 



