REPRODUCTION AND HEREDITY 121 



nucleus are derived through either parent, and from these 

 are derived by repeated fission all the chromatin in the 

 cells of the body whose development starts from the fertilised 

 egg. Clearly therefore the chromosomes are to be regarded 

 as furnishing the " mechanism of heredity " ; it is somehow 

 through them that " like begets like." 



But at the beginning of this chapter we reminded our- 

 selves that offspring may not resemble their parents exactly 

 and that members of the same family may differ from each 

 other. The process of reducing division in the maturation 

 of the germ-cells enables us, partially at least, to under- 

 stand how such differences, collectively known as variation, 

 are frequently an accompaniment of heredity, because the 

 behaviour of those minute chromosomes within the germ- 

 cell nuclei corresponds with facts that can be observed when 

 the characters of members of the families of successive 

 generations are compared. It is well known that this last- 

 named line of inquiry, pursued with regard to hybridised 

 varieties of plants by J. G. Mendel as long ago as 1865 

 (see W. Bateson, 1909), has been eagerly followed since the 

 beginning of the present century by many students of 

 various groups of animals, among which certain insects 

 have yielded most important results. 



A simple introductory example is afforded by the 

 " Orange " Moth {Anger ona prunaria) a common British 

 insect of the Geometrid family, the male of which has 

 orange and the female yellow wings, with delicate 

 darkish streaks scattered over the surface, a dark line 

 in the middle of the disc of each wing, and a series of 

 dark dots along the hinder edge or termen. There is a 

 form of this moth, known as the variety sordiata^ in 

 which the scattered dark streaks are reduced or absent but 

 there is on the forewing a great extension of the dark scaling 

 so as to form two conspicuous bands, one along the termen 

 and the other across the wing base (Plate IV, A). It has 

 been shown by L. Doncaster and G. H. Raynor (1906) that 

 if a pair of these moths, one of the pale type and the other 

 of the variety, be bred together, the offspring will show the 



