Distinction Between Species and Varieties. 585 
The truth of this conclusion has become more obvious 
owing to the attention which has been paid to the subject 
since the re-discovery of MENDEL'S work. The paral- 
lelism between the two groups of hybridization and the 
two types of systematic subdivisions has been most ex- 
haustively dealt with by TSCHERMAK, who attempted to 
base upon it a principle of distinction between the spe- 
cific hybrids and varietal mongrels. 1 
We will, then, regard the principle in its new form 
as demonstrated, and examine the question why the cri- 
teria which it supplies are not sufficient for universal 
application. In doing so I shall, for various reasons, 
leave the mutation crosses, which TSCHERMAK also re- 
gards as specific hybrids, out of consideration ; and shall 
denote the Mendelian hybrids as bi-sexual in accordance 
with MACFARLANE'S terminology, employing also the 
term uni-sexual in the sense in which it is used by that 
author. Expressed very briefly, therefore, bi-se.\'nal 
crosses produce varietal hybrids, uni-sexual ones spe- 
cific hybrids. 
But some limitation is necessary ; and herein lies the 
difficulty of the question, which is felt by every one who 
endeavors to apply the conclusions drawn from the study 
of hybrids to taxinomic problems. This limitation is, that 
the criterion really applies only to monohybrids ; for di- 
poly hybrids, however, only in so far as they can be com- 
pared with these. 
We have given the name of monohybrids to those 
mongrels whose parents differ from one another in a 
single elementary character only. Obviously they occur 
both in uni-sexual and in bi-sexual crosses. But, of 
1 E. TSCHERMAK, in the third appendix to his edition of MEN- 
DEL'S Versuche ilber Pflanzenhybriden, p. 58. 
