LECTURE VIII. 221 



ctaracterSj and compare these witli such as are considered 

 as decisive in similar species ; but they never think of insti- 

 tuting experiments in regard to the propagation of the dif- 

 ferent species ; and thus two naturalists, though perfectly 

 agreed about the definition of species, may entertain dilfei'ent 

 opinions as regards the application of the definition, so that 

 one assumes ten different species where the other only sees 

 one species with ten varieties. In living nature, the specific 

 character of propagation might be discoverable ; but in extinct 

 animals, — those thousands of species which have disappeared 

 from the surface of the earth and are only known to us from 

 their relics, — such proofs cannot be given ; and paleontology 

 would be deprived of a basis if species could only be deter- 

 mined by propagation, and not by distinctive characters. 



There can thus be no doubt that practically it is only the 

 distinguishing characters which must guide us ; whilst the 

 test of propagation can only be applied to man, the domestic 

 animals, and some others standing next to man. But when 

 we try to combine the distinguishing characters with the re- 

 sults of propagation, we meet with the most glaring contra- 

 diction, inasmuch as certain animals produce a fertile progeny, 

 and yet difier far more from each other than those which pro- 

 duce sterile hybrids. Giebel has scientifically demonstrated 

 that the races of dogs which produce a fertile progeny present 

 in size, hair, colour, the structure of the skeleton, the niimber 

 of toes, the formation of the skull and teeth, much greater 

 difierences than the horse and the ass, which produce sterile 

 hybrids. Such, therefore, as consider all dogs as races of one 

 species, must admit that, as regards the distinctive characters, 

 the races of some species may be more remote from each other 

 than species, — an admission which upsets all systematic natural 

 history. 



Species has been characterised as a permanent type ; and it 

 can easily be shown that such naturalists as assume this im- 

 mutability theoretically^ are practically forced to assume va- 

 rieties and races. Species has also been spoken of as an 

 original type, something primitive and fundamental ; and yet 

 it must be confessed that, in the histoiy of the earth, species 



