THE FOUNDATIONS OF GENETICS 271 



indicate the fundamental methods of transmission of heritable 

 characters. 



Methods of Study. The superficial facts of inheritance were 

 known long before there was a scientific foundation for their 

 interpretation. It requires no profound knowledge to establish 

 the fact that children have the traits of their parents, that some- 

 times they resemble more remote generations, and that individual 

 peculiarities are Hkely either to b(^ transmitted through many 

 generations or to appear onl}^ intermittently. In a practical 

 way man has taken advantage of this knowledge for the production 

 of various l)recds of domestic animals and many varieties of 

 plants. Hound and dachshund, mastiff and pekingese, are prod- 

 ucts of the same wolf-ancestors of many generations ago. Sweet 

 corn, flint, and dent varieties, as well as the primitive maize of 

 the American Indian, have been traced back to the wild teosinte 

 grass which now resembles them so slightly. Selection of animals 

 and plants showing the desired qualities and propagation exclu- 

 sively from these individuals have been responsible at least in 

 part for such changes. In other cases man has taken advantage 

 of the useful qualities of two species or varieties and combined 

 them by hybridization. The mule, a cross between the horse 

 and the ass, is such a hybrid. 



Galton's Laws. Before these principles became a recognized 

 part of scientific procedure attempts were made to formulate laws 

 of heredity, with partial success. Best known are the two laws 

 of Sir Francis Galton, who worked extensively with statistical 

 data on human characters. They are : 



The Law of Ancestral Inheritance. Each parent of an or- 

 ganism contributes one quarter of its inherited qualities, each of 

 its four grandparents one sixteenth, and so on. In other words, 

 the resemblance of an individual to all of its ancestors of any 

 generation is inversely proportional to the remoteness of the 

 relationship. 



The Law of Filial Regression. Variation of parents from the 

 racial mean is transmitted to offspring in a lessened degree (Fig, 

 156). Tall parents tend to produce tall offspring and short 

 parents short offspring, but the normal expectation is that these 

 offspring will be nearer average h(>ight than their parents. This 

 law is of practical value, but is subject to modification according 

 to facts discovered since Galton's time. 



