70 HEREDITY 



of certain -wild species of plants, but the results are not 

 at all convincing. 



In general, it seems that where we are dealing ^-ith 

 such characters as the excessive length of ear in the 

 " lop " rabbit, which have been produced by continuous 

 selection through many generations, we cannot expect 

 a simple MendeHan result. The end product of the 

 process of selection is probably a combination of many 

 factors, all having a certain effect on the character in 

 question, but independently heritable. The rebuilding 

 of this combination, once broken doA\Ti by crossing, 

 must necessarily be a difficult process, unless enormous 

 numbers of individuals are raised in the second hybrid 

 generation. We can, then, easily conceive causes which 

 would obscure the fact of segregation Mith regard 

 to such characters as ear length in rabbits and skin 

 colour in man. 



Another objection that has sometimes been put for- 

 ward to Mendel's law is that while segregation follows 

 in the main the MendeHan scheme, the " pure " types 

 extracted in the second hybrid generation are not 

 absolutely pure, but only relatively so ; that they 

 retain a trace of the hybrid character, and have a 

 tendency to " throw back " to it. 



In guinea-pigs, for instance, it has been found that 

 the ordinary short coat behaves as a dominant to the 

 so-called " Angora." The hairs of the " Angora " type 

 are very long, this condition being brought about by 

 the fact that they continue to grow indefinitely, whereas 

 the coat of an ordinary guinea-pig definitely ceases to 

 grow after it has reached a certain length. The length 

 of the " Angora " coat is, of course, also hmited, as 

 each hair ultimately dies and falls out. But in it there 

 is no definite stoppage of growth. A cross between 

 the two types gives a short-coated hybrid, and the 

 second generation gives segregation into three short- 

 coated, one Angora. But the short-coated individuals, 

 although the gro\\'th of their hair definitely ceases, are 

 sometimes distinctly longer coated than their short- 

 haired grand-parents. These individuals ^^^th a 



