ACCUMULATION OF NEW CHARACTERS. 927 



have been erroneously stated to occur, but the alleged hybrids are either true Pilo- 

 seiloideae or true Archieracia. . . . According to the present state of our knowledge, 

 no other hypothesis is possible but that all the various species of Hieracium have 

 sprung from the transmutation (descent with variation) of forms which have either 

 disappeared or are still in existence ; and a large number of the intermediate forms 

 •still occur which must have had their share in producing several new species by the 

 splitting up of ©ne original species, or which would have occurred in the transform- 

 ation of a still living species into one derived from it. In the case of Hieracium the 

 species have not become so completely separated by the suppression of the inter- 

 mediate forms as is the case in most other genera.' 



By the term Species is meant the aggregate of all the individual plants which have the 

 same constant characters, these characters being different from those of other somewhat 

 similar forms. It is clear from what has already been said that the only distinction 

 between varieties of a known primitive form which have become constant, and the wild 

 species comprised within a genus, is that in one case their descent is known, in the other 

 it is not. The various cultivated varieties of a primitive form which have become con- 

 stant are linked together by intermediate forms in which the progressive accumulation 

 of new varietal characters may be perceived ; but these intermediate forms may dis- 

 appear, and then there is a more or less wide chasm between the various varieties them- 

 selves, as well as between them and the primitive form. Both of these cases occur also 

 in wild plants. In some genera, like Hieracium, species the extreme forms of which 

 differ greatly are connected together by a number of intermediate forms which occur 

 along with them. The analogy of cultivated plants justifies us in considering these 

 intermediate forms (so far as they are not hybrids) as varieties in a progressive state 

 of development, some particular descendants of which have advanced furthest in the 

 accumulation of new properties. But usually the intermediate forms, the connecting 

 links so to speak between the ancestral and the derived forms, have disappeared ; and 

 the species of the same genus are then completely separated from one another, and their 

 characters are at once distinguishable. The different species of the same genus agree 

 among one another in a number of inherited characters, and are distinguished by 

 only a few constant characters ; the amount of resemblance is much greater than the 

 amount of difference. The same relationship therefore exists, but in a greater degree, 

 between the various species of one genus as between different varieties of the same 

 primitive form. Since no other explanation is known of this relationship than common 

 descent with variation and the heredity of the new characters, we are entitled to 

 consider the species of a genus as varieties of a common ancestral form which have 

 developed further and become constant,— the original form having possibly actually 

 disappeared or being no longer recognisable as such. There is therefore no natural 

 boundary-line between variety and species ; they differ only in the amount of divergence 

 of the characters and in the degree of their constancy. Just as a number of varieties are 

 included in the idea of a species — the varietal characters being neglected in the diagnosis 

 of the species — so several species are united into a genus by including in the diagnosis of 

 the genus a maximum of their common characters. But since it is impossible either to 

 determine by measure or by weight the most important characters of a plant, it is 

 ditficult and to a certain extent impossible to define, i.e. to determine by convention 

 what amount of differentiation is necessary in order to classify two different but similar 

 forms as species rather than varieties. In the same manner it is left to a great degree to 

 personal judgment to decide whether two different but icimilar groups of forms should 

 be regarded as two species each including several varieties, or as two distinct genera 

 each including several species. The only object actually presented to the eye is the 

 individual (and even this not always as a whole) ; the ideas Variety, Species, Genus are 

 abstract ideas, and indicate a progressive scale of the differences between individuals 



