1878 | Aspidium spinulosum and its Varieties. 711 
sia and under surface of the fronds are perfectly smooth, but in 
intermedium they are more or less glandular. 
This character appears to be constant and, therefore, reliable, 
but it is necessary to collect specimens early in order to observe 
it,as the glands are fugaceous, quickly disappearing after the indu- 
sium begins to contract—a fact which accounts for the difficulty 
oftentimes experienced in properly placing specimens collected 
date 
6. As to the position of the sori on the veins. If this character 
proves constant it will be the most important one of all, as it will 
enable us to place specimens collected at any time with a greater 
degree of certainty, and, in connection with the character of the 
indusium, enable us to clear away much of the confusion sur- 
rounding the different forms of the species. 
I sum up the relative value of the different characters as 
follows : 
1. Color—not constant. 
2. Scales—constant in ordinary forms of the species; not con- 
stant in the varieties 
3. Form—not constant. 
4. Structure—not constant. 
5. Character of the indusia—constant. 
6. Position of the sori on the veins—constant. 
It is too much to expect that any species or variety will exactly 
conform, in every instance, to any prescribed form or character, 
just as pieces of mechanism cast in dies by the hundred or thou- 
sand in unvarying conformity agree with one another; and all we 
can do is to describe the general characters of a species or variety 
as we find them in nature. 
A species often manifests itself in a great variety of forms, no 
two plants being exactly alike, and sometimes fronds on the same 
plant will exhibit surprising variation, so that it is exceedingly 
difficult to fix the limit and say which of the forms is typical, 
but as long as these forms can be referred to one common center 
of variation it does not seem well to recognize any such forms as 
varieties, and so we endeavor to describe as exactly as possible 
the character of that center of variation, and judge of all speci- 
mens by their relation to that. 
When we find plants that cannot be referred to this first center 
of characters, but evidently proceed from another, if there are no 
