CONTINUITY BETWEEN SPECIES 75 



of hybrids necessarily indicate that their parents belonged 

 to the same species, for here again the progeny of parents 

 belonging to obviously different species have been shown to 

 be perfectly fertile in some cases. 1 On the other hand, con- 

 siderable difference in characters between two races does not 

 necessarily constitute a species. 2 Sir E. Ray Lankester is 

 inclined to think that the word species should be abolished 

 altogether. 3 As, however, we are still obliged to use the 

 term, it is desirable to arrive at some definite idea. It is 

 not likely that any definition would be accepted by all 

 biologists. Probably it is not possible, without risk of con- 

 tradiction, to go farther than to say that forms which exhibit 

 " free interbreeding under natural conditions " and " have 

 been shown by human observation to be descended from 

 common ancestors or from a common parthenogenetic or 

 self-fertilising ancestor," 4 belong to the same species. Ad- 

 mitting this, we should imply that forms that do not inter- 

 breed freely under natural conditions and cannot be shown 

 by human observation to be descended from common 

 ancestors, should be classed as separate species. If we 

 add to these points of difference certain morphological 

 differences, the majority would admit that two forms were 

 rightly so classed. According to this there is no evidence 

 for the mutation theory, for there is nothing to show that 

 mutants will not interbreed freely with forms that exhibit 

 only fluctuating variations under natural conditions, and 

 the mutants have been shown to have the same ancestors as 



1 The hybrids between the common and the Chinese goose are fertile 

 (Life and Letters of Darwin, vol. iii. p. 240). These geese are "so distinct that 

 they have been placed by some authorities in distinct genera or sub-genera" 

 (Darwin). The hybrids from the ring-dove and domestic pigeon are also 

 fertile (P. St. M. Podmore, The Zoologist, November 1903, p. 401). See also 

 p. 118, for cases among fishes, and p. 117. 



2 e.fj. the case of the polymorphic butterfly Limnas chrysippus (Poulton, 

 Essays on Evolution, pp. 70-71). 



3 Stated by Poulton, op. cit., p. 62. 



4 Poulton, op. cit., pp. 60 and 61. (Presidential Address to the Entomological 

 Society of London, January 20, 1904.) 



