278 GENETICS 



swim, but sinks to the bottom and dies. The slow circula- 

 tion is incompatible with the possession of a large quantity 

 of yolk. 



In another respect the hybrids are in this case less fitted 

 to survive than the pure species. The hybrids are less re- 

 sistant to carbon dioxide than either parent species. The 

 hybrid is a less efficient physiological organization than the 

 parents. 



There are many cases in which there is thus a lack of 

 harmony in the development of hybrids, although it is not 

 always so clear as in the above case to what the disharmony 

 is due. Thus, Stockard reports that when the St. Bernard 

 dog is crossed with the Great Dane, the hybrids develop, 

 but become partially paralyzed when about three months 

 old. 



In some cases the hybrids develop to the adult condi- 

 tion, but harmony is lacking in the parts produced. These 

 are examples of the formation of new combinations of 

 genes and of characters produced by biparental reproduc- 

 tion, described in Chapter XII. The combinations formed 

 may be superior to those in the parents, or they may be 

 inferior. The latter is seemingly more frequently the case 

 when the two parent stocks are very diverse. There may 

 then be produced varied combinations of parts of different 

 form and size, some of them disharmonious. A consider- 

 able number of such cases are described by Stockard ^*' 

 among crosses of different breeds of dogs. An example of 

 such diverse combinations is the following. The short- 

 legged Basset hound is crossed with the long-legged Ger- 

 man Shepherd dog. In the later descendants (F2 genera- 

 tion), some have short legs, some have long legs, some 

 have legs of intermediate length (figure 57). Many ex- 

 amples of this type are given by Stockard. Not all are 

 clearly disadvantageous, though some seemingly are. 



In an example given by Lang,^^ a dachshund (legs very 



