atavisjn, but it is readily explained by Mendelian laws, as 

 follows : — 



Brown-eyed x blue-eyed Blue-eyed x brown-eyed. . . Pi 



Brown-eyed x Brown-eyed F, 



(impure) | - i impure). 



i 



Brown-eyed Brown-eyed Blue-eyed F> 



(pure)' (impure) (pure) 



Reversion is also sometimes explained by Mendelian 

 laws as latent characters appearing on segregation. Ex- 

 amples occur when varieties of white -flowered sweet peas 

 are crossed. (See Chapter 15). The progeny are purple 

 like the wild form; also when the Bush sweet pea is crossed 

 with the dwarf Cupid sweet pea (Punnett). Foals some- 

 times have a few stripes on the fore legs, and slaty-blue 

 pigeons occur among buff and white domestic pigeons. 

 Read Darbishire's account of the results of crossing Albino 

 mice with Japanese waltzing mice. 



Problems. — 1. Account for the occasional appearance of a 

 horned animal and of a red colored calf in 

 herds of Aberdeen-Angus. 

 2. Account for the occasional appearance of a 

 black hen in the progeny of Barred Ply- 

 mouth Rocks. 



(h)— Inheritance in Aphids or Plant Lice 



The life history of aphids is well known. The fertilized 

 egg hatches out a female, the stem-mother, the following 

 spring, followed by a succession of generations of females. 

 On the approach of autumn a brood of sexual males and 

 females appears. The explanation of these phenomena is 

 not clear, but some progress has been made in solving the 

 mystery. Morgan has shown that the spermatids of Phyll- 

 oxera are of two kinds, but those without an accessory 

 chromosome degenerate. Consequently only those con- 

 taining an accessory chromosome take part in fertilization, 

 and the fertilized eggs produce females. The problem of 

 the production of the males parthenogenetically, however, 

 at the approach of autumn has not yet been solved. It is 

 probable, however, that external or environmental factors 

 are to some extent responsible. In P. caryxcaulis one-stem 



126 



