Progression, Retrogression and Degression. 65 
I think it undesirable that these two types of sub- 
divisions of the species should continue to be denoted 
by the same term. The simplest plan would be to refer 
to the former as elementary species and only to the latter 
as varieties, and I hope that this limitation of the terms 
will come into general use. 
The question, however, is a purely systematic one and 
belongs to the department of descriptive science. For as 
soon as it is treated from the experimental standpoint 
the whole difference disappears. Many of the best vari- 
eties prove, when tested by sowing, to be as constant as 
elementary species, so that a separation on the basis of 
constancy is out of the question. 
# * # 
On the basis of the foregoing discussion I treat the 
homonomous subdivisions of the LINNEAN species as 
elementary species and eventually denote them with bi- 
nary names. In the case of derivative varieties, however, 
I prefer to make no definite choice ; I regard for example 
Chelidonium laciniatmn Miller and Chelidonlum inajus 
laciniatnni as equally justifiable. And when for instance 
several species in the same genus have white flowered or 
glabrous varieties, a binary nomenclature would obviously 
be much too cumbrous. 1 
7. PROGRESSIVE, RETROGRESSIVE AND DEGRESSIVE 
FORMATION OF SPECIES. 
A glance at the phylogeny of the vegetable kingdom 
reveals the fact that all species cannot have arisen in the 
same way. Progressive development is due to the con- 
dor instance if specific names like that of Agrostemma nicae- 
cnsis for Agrostemma Githago pallida were generally used for white 
flowered varieties. 
