shades, including the original types. The latter breed true, but the inter- 

 mediates continue to split up. After breeders had learned to count the num- 

 ber of individuals of each type that appeared in the progeny of hybrids, it 

 was easy to figure out that the color of wheat grain is inherited through the 

 combined effects of two or more "independent factors". This is in contrast to 

 the "single determiner" which Mendel assumed to account for each of the 

 seven pairs of contrasting characters in his garden peas. 



Perhaps we can get the idea of "multiple factors" from a more familiar 

 experience, that of variation in stature. In a group of men with an average 

 stature of (yl inches, some of the individuals are, let us say, only 63 inches tall 

 and others, say, lli inches tall. Variation in stature is "fluctuating" or con- 

 tinuous. We do not think of a special character "seventy-three-inchness" or 

 "sixty-four-inchness", but we do assume that "tallness" or "shortness" is 

 related to the heredity of the individual^ — that is, to something transmitted 

 from the parents. But the tallness, whatever it may be in a particular individ- 

 ual, is a composite made up of the x inches of the head, let us say, plus thejy 

 inches of the trunk plus the z inches of the legs. 



Charles B. Davenport (1866-1944), for many years director of the 

 Laboratory for Experimental Evolution of the Carnegie Institution of 

 Washington, suggested that stature is probably inherited as four (or more) 

 independent "factors" (see illustration, p. 484). That is, any segment of 

 "tallness" might be inherited independently of the others, according to the 

 Mendelian formula. Moreover, "long" might be dominant over "short" in 

 one segment and recessive in another. Some such supposition would help 

 to explain the familiar fact that children are sometimes shorter than both 

 parents, sometimes taller than both parents. 



It would also explain why the sons of a thousand tall fathers are taller (on 

 the average) than their contemporaries in general, but not as tall as their own 

 fathers, on the average — an illustration of Galton's "law of regression". 



The hundreds of experiments that agreed with Mendel's formulas, as well 

 as those that failed to match these formulas, made people wonder more and 

 more, Jus; \ow are the characteristics of plants and animals transmitted.'' 



What Is the Connection between Heredity and Reproduction? 



What Is Inherited? It is common to speak of the inheritance of charac- 

 ters as though something passed from parents to offspring. But a moment's 

 thought will show that nothing is transmitted in the ordinary literal sense. 



What we really mean by saying that a plant or animal has inherited certain 

 characters from his parents is that there is something in the fertilized egg that 

 brings about the development of those characters. But whatever is in the egg 

 must have come from the gametes, and so, presumably, from the parents. 



483 



