348 EVOLUTION AND GENETICS 



colonies become weak they may succumb to the inroads of the 

 bee moth. Hybrids between the two have no proved superiority 

 and retain the characteristic nervousness of the black bee which 

 makes them difficult to handle as compared with Itahans. In 

 addition they are the most ill-tempered of all three. Italians 

 vary greatly in temper but are in general mild, while the hybrids 

 sting readily under anj^ conditions. This cross is usually looked 

 upon as intervarietal, although the systematic rank of the various 

 kinds of honey-bees is by no means clear. 



Heterosis is sometimes as marked in animals as in plants. It 

 must not be thought, however, that because crossing may increase 

 vigor, inlDreeding must reduce it, for many strains of domestic 

 animals have been intensively inbred for many generations without 

 reduction of vigor or fertility. In some cases, in fact, these 

 qualities have been improved through inbreeding accompanied 

 by careful selection. In general, crossing results in heterosis while 

 inbreeding may be practiced with varied results, hence the con- 

 clusion has been reached that vigor and fertility depend upon 

 unit characters which are much more likely to be isolated in 

 homozygous combinations through inbreeding than through the 

 mixing of various strains. 



Limitations of Hybridization. The crossing of different 

 species is attended l)y many difficulties which limit its value. 

 Some are insurmountable while others can be met by special 

 methods of procedure. Hyl)ridization within a species, whether 

 between varieties or with respect to single characters, is a much 

 simpler process. 



Interspecific infertility is the most serious of these limitations, 

 for when normal union of the germ cells cannot take place 

 no hybrid can be produced. It is such a common phenomenon 

 that some biologists have recognized in it a criterion for the 

 limitation of species. While such an extreme interpretation is 

 not favored by the availal^le evidence, the fact remains that many 

 species cannot be crossed. The facility with which hybridization 

 may be accomplished is often, if not always, in proportion to the 

 degree of relationship between the species involved. 



The reasons for infertility are various. In some cases the sperm 

 cell is unable to penetrate the surface of the ovum of a different 

 species. In others the spermatozoon not only enters the ovum 

 but also initiates development, although it makes no material con- 



