48 EVOLUTION OF TO-DAY. 



The most common is the crossing of two quite dif- 

 ferent individuals. If, for instance, two very differ- 

 ent breeds of pigeons be crossed, there is a great 

 tendency to produce young quite unlike either par- 

 ent, but resembling the rock pigeon, which is known 

 to be the ancestor of our races of domestic pigeons. 

 This tendency to reversion can always be found 

 more or less marked in domestic races. It is cer- 

 tainly a marvellous thing that characteristics which 

 have been lost for many generations should have 

 thus the power to reappear. Science has no ex- 

 planation to offer for the fact. Such reversions are 

 never supposed to give rise to new varieties. They 

 are, on the other hand, claimed to prevent the 

 formation of new varieties. It is said by those 

 who do not accept the unstability of species, that 

 these reversions keep the species true, by con- 

 tinually bringing the individuals back to the cen- 

 tral type when they depart too widely from it. 

 Reversion has, however, never been known to pro- 

 duce such a result. It does not take place in ordi- 

 nary circumstances, but only when the great changes 

 occur. 



But these reversions are frequently of great assist- 

 ance to the naturalist in enabling him to trace the 

 history of varieties. When it is found that all of 

 our domestic pigeons revert in this manner to the 

 rock pigeon, it is almost sure proof that this rock 

 pigeon was their common ancestor, even if this fact 

 stood alone. Now the important point to be noticed 

 is, that reversions of a precisely similar character occur 

 among the species of a genus. For instance, when 



