VARIATION AS THE BASIS FOR HEREDITARY DISTINCTIONS 317 



lem. Actually any organism is the product of both heredity and environ- 

 ment. We can produce variations either by keeping the heredity uniform 

 and varying the environment or by keeping the environment uniform and 

 varying (by crossing) the heredity, and many phenotypic qualities are 

 demonstrably due to the coincidence of a particular genotype with a 

 particular environment. 



This is very clearly illustrated by the case of a garden flower, a red 

 Chinese primula. In the primulas, there is a red race that, when crossed 

 with a white race, gives a pink (pale red) F\ and an F 2 with the ratio 34 

 red, x /i pink, and 34 white — a typical monohybrid in which dominance 

 is lacking. But if the red-flowered plant is taken from the garden and 

 grown in a hothouse at a temperature of 95°F. or more, its flowers will 

 be white. If 10 generations are kept continuously at this high temperature, 

 they will produce nothing but white flowers in all this time; yet if the 

 eleventh generation is returned to the garden, the flowers will again be 

 red. Here the gene that distinguished the red-flowered variety from the 

 white-flowered variety at lower temperatures is not a gene for red but a 

 gene to produce red at normal temperature and white at temperatures of 

 95°F. and above. 1 



Many other genes are known that produce one quality in one environ- 

 ment but fail to produce this quality, or produce a different quality, in a 

 different environment. There are also many other characters for which 

 the phenotypic expression of the gene is dependent on the physiological 

 condition (internal secretion) of the organism itself. This is well illus- 

 trated in the secondary sexual characters of animals. The natural type 

 and color of a man's beard are inherited (through the mother as well as 

 through the father), but if a man is castrated, he fails to have a beard, 

 and although a woman has factors for a beard — as is shown by her trans- 

 mitting to her sons the factors for her father's type of beard — she does 

 not develop a beard. The same phenomenon is seen in the inheritance 

 and appearance of horns in certain races of sheep and in the inheritance 

 of the male plumage in birds. A very similar condition is seen in the case 

 of at least certain types of goiter in man, where the development of goiter 

 is dependent upon both genetic factors and the amount of available 

 iodine in the environment. 



What is inherited? In view of the foregoing considerations and of 

 what we have seen of Mendelian inheritance, we can define inheritance 

 as the transmission from generation to generation of a tendency to react 

 in a certain way to (1) a given environment, (2) a given physiological 

 constitution, and (3) a given complex of other hereditary factors; or, 

 better, since (2) and (3) are also inherited, as the transmission of a tendency 

 to react in a certain way to a given environment. 



1 Doubts that were cast upon this interpretation of the primula case have proved 

 unfoimded. 



