mDIVIDUALITY AND HTBBIDITY. 657 



of free motion and of maintaining an independent existence. 

 So with many other Ccelenterates and with the tapeworm,, 

 whose proglottides or segments are finally capable of sepa- 

 rate existence. Among the higher invertebrates, even the- 

 different members of a colony of white or true ants lack a. 

 certain amount of individuality, the workers performing 

 labors upon which the maintenance of the very existence of 

 the colony depends, so that there are different grades of in- 

 dividuality, from examples like the Hydractinia and the 

 Siphonophores up to those insects which live socially ; and 

 we see that the most perfect individuality exists in those- 

 animals which can most efficiently provide for their own 

 sustenance and for the continuance of their species. 



Hytaridity.— It is rare that two species, even of the same 

 genus, can produce offspring ; when such cases occur, the 

 result is called a hybrid. For example, the mule is a hybrid, 

 being bred from a female horse and an ass ; but the mule 

 is not fertile, and hybrids are very rarely fertile. The In- 

 dian dog and coyote are said by Coues to interbreed, and 

 on the Upper Missouri we have seen dogs which had every 

 appearance of being such hybrids. Dogs also cross with the 

 fox (Darwin). The American bison is known to breed with 

 the domestic cattle, and it seems to be a well-established 

 fact that the hybrids are fertile. Fish readily hybridize. 



Darwin states that he knows of no thoroughly well-au- 

 thenticated cases of perfectly fertile hybrid animals, though 

 he adds, " I have reason to believe that the hybrids from 

 Cervulus vaginalis and Reevesii and from Phasianus ml- 

 chicus with P. torquatus are perfectly fertile." The hare 

 and rabbit are supposed to have fertile offspring ; the hy- 

 brids of the common and Chinese geese {Anser cygnoides) 

 are fertile. The crossed offspring from the Indiai humped 

 and common cattle interbreed. Cuton has hybridized the 

 Virginia deer with the Ceylon deer and the Acapulco deer ; 

 " the hybrids seem perfectly healthy and prolific." Among 

 insects over 100 cases of hybridity have occurred. Hybrids 

 between the brown and polar bear, the leopard and jaguar, 

 Equus onager and E. hemippus, E. hurchelK with the com- 

 mon horse, and with the common ass and E. hemionus ; 

 have been niispd. 



