n8 THE FIRST PRINCIPLES OF HEREDITY 



Altogether, the same considerations will weigh that we have 

 already discussed in the crossing of different strains. The 

 following diagram, given by Professor Ziegler, will make the 

 matter clear : 



Father with marked taint, inherited from his father 

 and mother, as shown by the dark chromo- 

 somes 13 out of 24. 



O OO 



Mother normal, though with a latent taint, inherited 

 from her mother, as shown by the dark 

 chromosomes 4 out of 24. 



OOO OOO OOO OOO O O OOO OOO 



Three mature egg-cells f * O O O OOO OOO 

 showing three different -I e. O OOO OOO OOO 

 combinations [/ O O O OOO OOO OOO 



FIG. 46. INHERITANCE OF DISEASE. (After Ziegler.) 

 (From J. A. Thomson, "Heredity.") 



IV. -CONSANGUINITY. 



There is a general impression, since Darwin has shown 

 that continued inbreeding among plants and animals has 

 a deleterious effect on their fertility and general vigour, 

 that marriages among near kin are unhealthy, and therefore 

 to be avoided. Prevalent though this idea is, it is hardly 

 borne out by scientific facts. It is true that, according to 

 some investigators, signs of degeneration have been found 

 in certain closely inbred populations, but, on the other 

 hand, in other cases no such bad effects could be found. 

 The evidence, as far as it stands at present, is not con- 

 clusive either way. All that can be said on this question 



