ALLEN, — THK HIOUKDITY OF COAT COLOR IN MICE. 71 



complete albinos at this stage, but none of this sort have been obtained 

 in these exjieriinents. 



Mice may be mated at the age of three months, and they will continue 

 to breed until a year old or slightly longer. But although they may live to 

 be 15 or 18 mouths old, they are much less prolific in the latter months 

 of their life. Moreover, in the writer's experience, they rarely breed so 

 often as once in three weeks, although they are capable of so doing. 



IV. Mendkl's Laws of Herkdity. 



Mendel's great discovery of the principles in heredity of dominance and 

 segregation of characters is now so generally known as hardly to need 

 extensive notice here. These two important laws were first set forth by 

 Mendel in 18G6, though for nearly half a century they passed quite 

 unnoticed. In 1900 the botanists de Vries and Correns, working in- 

 dependently of each other, again brought to light Mendel's principles, 

 and were able to give them further confirmation through their own 

 experiments. The three following years have seen a keen and growing 

 interest in the field thus opened, and already a considerable body of 

 literature has appeared for or against the ^Nlendelian laws. A full ex- 

 position of the Mendelian doctrine may be found in the works of 

 Bateson (: 02) and Castle (: 03^ :03''), and it is from these that the 

 following brief statement is principally drawn. 



In his experiments with peas, Mendel established the fact that when 

 two varieties are crossed, whose seeds are respectively yellow and green, 

 the hybrid seeds produced are yellow, that is, the yellow color of cotyle- 

 don is dominant, while the green color does not appear in the cross-breds, 

 though it will be represented in their germ cells. If D represent the 

 dominant character and R the recessive character set off against it, we 

 may express Mendel's result by this formula (1): DxR = 4D(R),* 

 the parenthesis indicating that the enclosed character is present in the 

 germ cells but does not appear in the soma. In such cases of simple 

 dominance, the first filial generation, F,,! resembles in external appear- 

 ance the parent possessing the dominant character. That this resemb- 

 lance does not signify identity in character may be shown by breeding 

 inter se the D(R) hybrids obtained from the above cross, Mendel 

 proved that for his peas the hybrids, D(R), produced germ cells D and 



* In this equation the symbols at the left of tlie sign of equality indicate the char- 

 acter of tlie parents, those at the ritrht tlie cliaracter of their offspring. 



t Tliis convenient notation, suggested by Bateson (:02), will be employed 

 throughout tliis paper/ 



