134 SECRETS ^OF EARTH AND SEA 



species of flat-fish, such as the turbot and the brill, though 

 it is difficult to be sure that the rare hybrids so produced 

 are fertile even if they attain to maturity. The same is 

 true as to certain small flowering plants having distinct 

 regions of natural distribution and occurrence. At the 

 confines of the regions proper to two such allied species, 

 insects passing from one to the other do sometimes effect 

 a reciprocal fertilization of the two species, and a natural 

 hybrid is the result. Here, again, it is difficult to follow 

 the subsequent history of the hybrids, but it is believed 

 that in some instances they are fertile, and that the 

 hybrid race is only gradually merged by subsequent 

 crossing into one or other of the parent species. Not 

 a single instance is on record of the production of a 

 " natural " hybrid (that is to say, one produced in natural 

 conditions without man's interference), whether fertile or 

 infertile, between two species of the larger animals (such 

 as between horse and ass or zebra and ass, or between 

 lion and tiger or any of the species of cats, or between 

 species of bears) or birds (such as pheasants of various 

 species, including the jungle cock, the wild original of 

 our domestic fowl, or between various species of ducks, 

 various species of geese, or between various species of 

 the grouse-birds). 



Nevertheless, in conditions brought about by man — 

 that is to say, confinement in cages or paddocks, or at 

 any rate removal from their native climate and home — 

 all the groups of species just cited commonly and fre- 

 quently produce hybrids inter se, that is, one or more 

 species of the horse group thus inter-breed with one 

 another, so will certain species of cats, certain species 

 of bears, many species of pheasants, also of ducks, of 

 geese, and of grouse. In nearly every case the hybrids 

 so produced are infertile : they will not mate with a 

 similar hybrid, and even when mated with one of the 



