74 HEREDITARY CHARACTERS 



follow him ; but it is the continuity which rather aggres- 

 sively impresses the great majority of those whose lives 

 are devoted to the study of species. The work of the 

 systematist would be immensely facilitated by that very 

 discontinuity which is always eluding him but obtrudes 

 itself upon Bateson. The letters of Darwin ... are almost 

 pathetic in their statement of difficulties due to continuity in 

 Cirrhipedes." * Rothschild and Jordan write : " Whoever 

 studies the distinctions of geographical varieties closely and 

 extensively, will smile at the conception of the origin of 

 species per saltum" 2 



It is quite clear, then, that many varieties and so-called 

 species do merge into each other, or at any rate that they 

 are not sharply marked off from each other. It is apparent, 

 however, that different writers mean different things by the 

 term "species," or at any rate have different ideas as to 

 what constitutes a species. It is impossible to deal here 

 with the question, " What is a species ? " It is necessary, 

 however, to deal with some points very briefly, in order 

 to arrive at a proper conception of what is claimed for the 

 mutation theory. It seems fairly obvious that the property 

 of breeding true does not constitute a species or even a 

 variety, even when there are considerable differences between 

 several races. Local groups of individuals differ considerably 

 from other similar groups in other places, but no one would 

 claim for them the rank of a variety, let alone a species. 

 Such groups generally breed true. Yet breeding true has 

 been classed very recently as " one of the crucial tests of 

 specific or sub-specific rank." 3 Neither can it be admitted 

 that fertility between two divergent forms necessarily proves 

 that they belong to the same species. Hybrids between indi- 

 viduals of obviously different species and even different genera 

 have frequently been produced. 4 Neither does the fertility 



1 E. B. Poulton, Essays on Evolution, pp. xiv, xv, Oxford, 1908. 



2 Novitatcs Zooloyicce, vol. x. p. 492, 1903. 



3 Thomson, 1908, op. cit., p. 92. 



4 See pp. 75, 117, 118. 



