112 ]NTKODUOT10N. 



XIV. Hybeids. 



The fact, so often observed, tliat pairing takes place 

 between different species — as, for instance, between 

 Rana temporaria and Biifo vidgaris, Rana temporaria 

 and Pelobates fiiscus, Pelodi/tes punctatus and Ilijla 

 arborea — has long ago given rise to the supposition 

 that hybrids are occasionally produced. At the end 

 of the last century, Spallanzani, after his successful 

 experiments on artificial fecundation, attempted to 

 raise hybrids between species of Rana, Bufo, and 

 Hj/la, but without results. 



Experiments wei^e again instituted, from 1867 to 

 1872, by A. de I'lsle, who tried to hybridise the 

 French species of Rana and Bufo. The difficulty of 

 such experiments is very great, owing to the diversity 

 in the breeding season of the different species. Never- 

 theless de I'lsle succeeded in procuring suitable ex- 

 amples of three species of Rana, and artificially 

 impregnated ova of Rajta esculenta and R. agilis with 

 the semen of B. temporaria, and ova of Rana tem- 

 poraria with the semen of the two others. The ex- 

 periments, several times repeated, gave a negative 

 result, which the author attributes to the very con- 

 siderable difi"erence between the genital organs of even 

 so closely allied species as R. temporaria and R. agilis. 

 But with Bufo vulgaris and B. calamita, which, 

 although morphologically more widely separated, 

 have almost identical genitalia, positive results were 

 obtained, although the development did not proceed 

 beyond the tadpole condition. 



A few years later, in 1877 and 1878, the subject 

 was again taken up by Lataste, who succeeded in 

 fertilising eggs of Pelobates fuscus with the semen of 

 P. cultripes. But the few tadpoles obtained were 

 monstrous or dropsical, and not one survived to 

 develop the limbs. 



