The Practical Conception of Species. 591 
ical and critical arguments for this practice. The word 
stirp would perhaps correspond most closely to the Ger- 
man word Sippe } 1 although this word has been employed 
by various authors in a different sense. 1 ' At any rate it is 
most desirable to make some distinction of this kind, and 
BELLI'S suggestion might well form the foundation of 
such. But questions of nomenclature have little interest 
in this discussion and I prefer to leave the task to others. 
\Ye may now proceed to the practical delimitation of 
the conception of species. 
Descriptive biology wants a definition, independent 
of the results of hybridization ; in its commonest form it 
is based on the absence or presence of transitional forms, 
as was explained in the first part of this volume. Groups 
of individuals which are connected together by transitions 
are considered to belong to the same species. The limits 
between species correspond to gaps in the series. 3 With- 
out some such convention the description of species oc- 
curring in collected material would be impossible ; and this 
method has been employed by the best systematists since 
the time of DECANDOLLE. It is only when direct experi- 
ments can be carried out that the problem can be dealt 
with in a different way. 
Two difficulties, however, present themselves, which 
T shall now briefly deal with. In the first place, exactly 
1 Die Mutatwncn nnd die Mutationsperioden bei dcr Entstehung 
dcr Art en. Leipsic, VEIT & Co., 1901, p. 14. 
2 C. CORRENS, Schcinbarc Ausnahmcn von dcr MendeVschen Spal- 
lungsrcgcl fiir Bastardc, Bcr. d. d. hot. Ges., 1902, Vol. XX, Part 3 ; 
p. 170. See also the same author in Ber. d. d. hot. Ges.. 1901. Vol. 
XIX, p. 77, note i, and his Monographic dcr Maisbastarde, p. i, and 
VON WETTSTETX. Grundziige dcr geographisch-morphologischen Mc- 
thodc dcr Pflanzcnsystcmatik , 1898, p. 3. 
3 See, for instance. Anncc biologiquc, IV. 1898, p. 470; V, 1899, 
p. 377 and elsewhere. Also BORRADAILLE, On Crustaeians, 1901, p. 193; 
GRISEBACH, Die 1 7 egetation dcr Erdc nach Hirer kh'niatisehen Anord- 
ining, 1872. p. 9. and so forth. 
