270 PHYSICAL BASIS OF HEEEDITY 



a number of small differences. The student of Men- 

 delian heredity at least is not likely to fall into the error 

 of identifying single Mendelian differences with the sum 

 total of differences by which wild types and often even 

 wild varieties differ from each other, but whenever he 

 has had an opportunity to study these single differ- 

 ences in wild varieties he has found that they seem to 

 originate and to be inherited in the same way as other 

 Mendelian characters. , 



Species as Gkoups of Genes 



If related species have many genes in common they 

 may be expected to produce at times the same mutants. 

 In fact, it is not at all uncommon to find even in Men- 

 delian literature such forms as albinos spoken of as though 

 they represent the same mutation wherever it arises. 

 Attractive as such a view appears, experience has shown 

 that it is very unsafe to judge as to the nature of the muta- 

 tion from the appearance of the character alone. Two 

 different white-flowered races of sweet peas are known 

 which give the wild purple-flowering pea when crossed, 

 showing that they represent different mutations. Simi- 

 larly, at least two recessive white races of fowls are 

 known, as well as a third dominant white race. Three 

 independent mutations have produced white birds. 

 Whether albino mice, rats, rabbits, squirrels and guinea 

 pigs have arisen through a mutation in a common gene 

 cannot be determined because they cannot be crossed 

 to each other. When we consider that many factors may 

 combine to produce a given pigmented animal, and that a 

 change in any one of them may affect the end result, it 

 will be evident that the expectation would be against 

 rather than for the conclusion that the same gene had 

 changed in all cases. Only when it could be shown that a 

 particular gene of the complex is more likely to change 

 in a given direction than other genes of the complex would 

 this interpretation become plausible. 



