ACCUMULATION OF NEW CHARACTERS. 
927 
have been erroneously stated to occur, but the alleged hybrids are either true Pilo- 
selloideae 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 one 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 
difficult 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 similar 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 
