148 A SCIENTIFIC APPRECIATION [ch. vi. 
but there is a distinct departure from the hard and 
fast idea of the fixity of species. Indeed he goes 
the length of saying, “We have no certain data 
from which we can infer the general permanence of 
specific forms.” 
After defining a species as “the aggregate of 
individuals agreeing in all their essential characters, 
breeding freely together, and producing perfect 
seed, which gives rise to similar individuals, also 
breeding together,” he goes on to say: “ There is 
nothing absolutely certain as to species, much less 
as to the groups into which they are disposed, as 
genera, families, orders, tribes, and the like. We 
merely agree to consider as species individual 
plants which closely resemble each other in the 
structure and form of their organs. Such species, 
however, often pass into each other by gradations, 
which render it impossible to draw a line of 
demarcation, and thus all species are more or less 
arbitrary. We know from observation, that all 
assumed species undergo changes from climate, 
cultivation, and other influences .- 1 and individuals 
exhibiting remarkable alterations we call collec¬ 
tively varieties; but variety is a still more vague 
idea than species.” 
It is interesting to find that the subject of 
variations had a particular attraction for Mac- 
Gillivray, and formed the theme of three articles 
in the Edinburgh Journal of Natural History (1837) 
which he edited. He there distinguishes minor 
1 Italics are ours. 
