70 THE PRINCIPLES OF HEREDITY 



which have arisen along the diverging lines of evolution, 

 tend to disappear. The ancestral form common to both 

 varieties, even if it be separated from the crossed descendants 

 by thousands of generations, tends to reappear. Sometimes 

 the reversion is direct ; the ancestral form reappears imme- 

 diately in the offspring of the cross. At other times the 

 offspring blends in the first generation, but reverts in 

 subsequent generations. Or the descendants of the blend 

 may revert, not to the remote common ancestor, but to one 

 or other of the crossed varieties ; the inheritance in this case 

 being exclusive. Now if bi-parental evolution were a cause 

 of progressive variations we ought to find that great pro- 

 gressive variations commonly result from cross-breeding; 

 for here the germ-plasms are extremely dissimilar. Instead, 

 we find evidence of great regressive variations only. In no 

 case is there the least evidence that bi-parental reproduction 

 is the cause of progressive variations. We may quote a few 

 examples. 



117. If we mate a Burchell zebra with a horse, "some of the 

 hybrids in make and disposition strongly suggest their zebra 

 sire, others their respective dams ; but even the most zebra- 

 like in form are utterly unlike their sire in their markings. 

 It is not a matter of taking after a grandparent, but after an 

 ancestor thousands of generations removed, an ancestor pro- 

 bably far more like the Somali than any of the Burchell 

 zebras ; . . . even when the hybrids are distinctly horse-like 

 they never repeat recently acquired peculiarities, such as a 

 blaze or short ears, high withers, or a small head and long 

 neck." i 



118. " Although mules are not nearly so numerous in Eng- 

 land as asses, I have seen a much greater number with striped 

 legs, and with the stripes far more conspicuous than in either 

 parent form. ... In South America, according to Roulin, 

 such stripes are more frequent and conspicuous in the mule 

 than in the ass. In the United States, Mr. Gosse, speaking of 

 these animals, says, c that in a great number, perhaps in nine 

 out of ten, the legs are banded with transverse dark stripes/ " 2 



119 " Professor Jaeger has given a good case with pigs. He 

 crossed the Japanese or masked breed with the common 

 German breed, and the offspring were intermediate in char- 

 acter. He then re-crossed one of these mongrels with the 

 pure Japanese, and in the litter thus produced one of the 

 young resembled in all its characters the wild pig ; it had a 



1 The Penicuik Experiments, p. xii. 

 Animals and Plants, vol. ii., p. 16. 



