142 MEANING OF THE SYSTEM. 



practice. This relation has been excellently and thoroughly discussed 

 by Darwin and Hooker. As an example of the difference of opinion 

 on this subject, Nageli * divided the Ilieracice found in Germany 

 into three hundred species, Fries into one hundred and six, Koch 

 into fifty-two, while other authors recognise hardly more than twenty. 

 Nageli indeed says, " There is no genus of more than four species on 

 which all botanists are agreed, and many examples may be cited 

 in which, since Linnaeus' time, the same species have been repeatedly 

 divided up and re-united." 



We are therefore driven, in order to determine the essential pro- 

 perty distinguishing species and variety, to consider the most impor- 

 tant characteristic for the conception of species, a characteristic 

 which has hardly ever been used in practice, i.e., the community of 

 descent and the capacity for fruitful interbreeding. This means of 

 determination, however, is also insufficient. 



It is a commonly known fact that animals which belong to different 

 species pair with one another and produce hybrids, e.g., horse and 

 ass, wolf and dog, fox and dog. Widely differing species, which are 

 placed in different genera, have even been known to cross with one 

 another, and to produce progeny, such as the he-goat and sheep, and 

 the she- goat and ibex. The hybrids however are, as a rule, sterile. 

 They are intermediate forms with imperfect generative system, with- 

 out the power of propagation ; and even in those cases where there 

 is a power of reproduction (such cases are most frequently met with 

 amongst female hybrids), there is a tendency to revert to the 

 paternal or maternal species. 



There are, however, exceptions to the sterility of the hybrid which 

 appear to afford weighty proof against immutability of species. The 

 experiments in breeding between the hare and rabbit, made on a 

 large scale in Angoulerne by Roux, have shown that their progeny, 

 the hare-rabbit, is perfectly fertile. Half-bred hybrids of the rabbit 

 and hare have been bred, and have been reproductive through many 

 generations of pure in-breeding. In like manner careful inquiries 

 into the hybridism of plants, especially the investigations of W. 

 Herbert, lead to the conclusion that many hybrids are as perfectly 

 productive among themselves as genuine species. 



In a state of nature, too, hybrids of various kinds are found. 

 Such hybrids have frequently been taken for independent specie?, 

 and have been described as such (Tetrao medius, hybrid of Tetrao 



* C. Nageli, " Entstehung uncl Begriff der naturhistorischen Art." Munich, 

 1865. 



