THE TAXONOMIC UNIT 
43 
Johannes Muller and De Quatrefages followed in the same line 
of thought. The former referred to a species as “a living form 
represented by individual being, which reappears in the product of 
generation with certain invariable characters, and is constantly 
reproduced by the generative act of similar individuals.” While 
De Quatrefages defined species as ‘‘an assemblage of individuals 
more or less resembling one another, which are descended or may 
be regarded as being descended, from a single pair by an uninter- 
rupted succession of families.” In these earlier years the con- 
ception of species was dominated by the principles of immutability 
and discontinuity. 
More recently there has been a tendency to emphasize the value 
of physiological functions in the diagnosis of species and varie- 
ties. This seems to be an especially easy point of view for the 
student of bacteria and smaller fungi. The metabolic processes 
of the bacteria, for instance, seem to be more readily distin- 
guished, if not more constant, than the structural peculiarities. 
And, of course, a very excellent case can be made out for the 
specificity of such physiological characters. It must be borne in 
mind, however, that back of every physiological process there 
must be a morphological organization which carries the same spe- 
cific peculiarity. The same may be said of peculiar and charac- 
teristic secretions, such as gums, oils, alkaloids, etc. 
During the early part of the preceding generation there was a 
trend away from the Linnaean conception of species. Thus, 
Huxley expressed his conception of species in this language: 
“When we call a group of animals, or of plants, a species, we may 
imply thereby, either that all these animals or plants have some 
common peculiarity of form or structure; or we may mean that 
they possess some common functional character.” 
Haeckel says that the word species “serves as the common desig- 
nation of all individual animals or plants which are equal in 
all essential matters of form, and are only distinguished by quite 
subordinate characters.” 
In this latter group of definitions we will observe that the 
principle of structural similarity is dominant. Nothing can be 
more evident to the biologist, who is compelled to deal, even super- 
ficially, with the nomenclature of organisms, than that fundamen- 
tal concepts and terms are in marked process of change. Half 
a century ago Professor Owen remarked : “I apprehend that few 
naturalists nowadays, in describing and proposing a name for 
