120 Heredity. 



does in the offspring of parents of the same variety. It 

 is well known that this is the case; that, in domesticated 

 animals and plants at least, crossing is a great cause — 

 according to some older writers the only cause — of varia- 

 tion. 



Darwin says that it is probable that the crossing of two 

 forms when one or both have long been domesticated or 

 cultivated, adds to the variability of the offspring, inde- 

 pendently of the commingling of the characters derived 

 from the two parent forms. He believes that new char- 

 acters arise in this way in hybrids between domesticated 

 forms, forms which have been rendered variable throngli 

 cultivation, but he doubts whether we have, at present, 

 sufficient evidence to prove that the crossing of species 

 which have never been cultivated leads to the appearance 

 of new characters. 



The following illustrations of this law are quoted from 

 his Variation (Vol. ii. p. 319): 



'* Gartner declares, and his experience is of the high- 

 est value on such a point, that when he crossed native 

 plants which had not been cultivated, he never once saw 

 in the offspring any new character; but that from the 

 odd manner in which the characters derived from the 

 parents were combined, they sometimes appeared as if 

 new. When, on the other hand, he crossed cultivated 

 plants, he admits that new characters occasionally ap- 

 peared. . . . According to Kolreuter, hybrids in the ge- 

 nus Mirabilis vary almost infinitely, and he describes new 

 and singular characters in the form of the seeds, in the 

 colors of the anthers, in the colyledons being of immense 

 size, in new and highly peculiar odors, in the flowers 

 expanding early in the season, and in their closing at 

 night. With respect to one lot of these hybrids he re- 

 marks that they presented characters exactly the reverse 



