FEBTILITY OF HYBRIDS. 143 



wrogattus and Tetrao tetrix ; Abramidopsis LeucJcarti, Bliccopsts 

 abramorutilus, and others are, according to von Siebold, hybrids.) 

 Sterility of hybrids is not the rule here, for a great number of wild 

 plants have been recognised as hybrid species (Kolreuter, Gartner, 

 Nageli Cirsium, Cytisus, Rubus). This seems to render it the less 

 doubtful that amongst animals which have been domesticated by 

 man, persistent transitional forms can be obtained from originally 

 different species, by gradual alteration brought about by cross 

 breeding. 



Pallas, adopting this view, gave it as his opinion that closely allied 

 species, though at first they may refuse to breed together, or may 

 produce sterile offspring, will, after long domestication, produce fertile 

 progeny. And in fact, it has been shown to be probable that some 

 of our domestic animals have originated in prehistoric times as the 

 result of the unintentional crossing of different species. Riitimeyer 

 especially endeavoured to prove this mode of origin for the domestic 

 ox (Bos taurus), which he regarded as a new race resulting from the 

 crossing of at least two ancestral forms (Bos primigenius, brachyceros). 

 It may be looked upon as certain that the domestic pig and cat, and 

 the numerous breeds of dogs, have originated from several wild 

 species. 



In connection with the exceptional cases which have just been 

 discussed, we may lay great stress upon the perfect reproductive 

 capabilities of mongrels, that is, of the progeny produced by the 

 crossing of different varieties of the same species ; though here also 

 we meet with exceptions. Disregarding those cases in which me- 

 chanical causes render the interbreeding of different varieties im- 

 possible, it seems, according to the observations of breeders whose 

 word may be depended upon, that certain varieties have difficulty 

 in crossing with one another ; and further that certain forms which 

 have been bred by selection from a common stock are altogether in- 

 capable of fertile intercourse. The domestic cat introduced into 

 Paraguay from Europe has, according to Rengger, become essentially 

 altered in process of time, and has acquired a marked aversion to 

 the European ancestral form. The European guinea pig does not 

 breed with the Brazilian form, from which it is probably descended. 

 The Porto-Santo rabbit, which was exported from Europe to Porto- 

 Santo near Madeira in the fifteenth century, is so much altered that 

 it can no longer breed with the European race of rabbits. 



The evident difficulty of precisely defining the conception of species, 

 in presence of the existence of a gradual, almost uninterrupted series 



