36 ASPECTS OF BIOLOGY AND METHOD OF BIOLOGICAL STUDY. 
ancestor, and the Descent Theory is simply the extension to the 
larger groups of this same idea of relationship. 
If this be a true principle, then biological classification becomes 
an exposition of family relationship—a genealogical tree in which 
the stem and branches indicate various degrees of relationship and 
direct and collateral lines of descent. It is this conception which 
takes classification out of the domain of the purely morphological. 
Affinity determined by the study of Anatomy and Development.— 
From what has just been said it follows that it is mainly by a com- 
parison of organisms in their anatomical and developmental char- 
acters that their affinities are discoverable. The structure of an 
organism will in by far the greater number of cases be sufficient to 
indicate its true affinity, but it sometimes happens that certain 
members of a group depart in their structure so widely from the 
characters of the type to which they belong, that without some 
other evidence of their affinities no one would think of assigning 
them to it. This evidence is afforded by development. 
A Philosophical Classification cannot form a single Reetilineal 
Series. —A comparison of animals with one another having thus 
resulted in establishing their affinities, we may arrange them into 
groups, some more nearly, others’ more remotely related to one 
another. The various degrees and directions of affinity will be 
expressed in every philosophical arrangement, and as these affini- 
ties extend in various directions, it becomes at once apparent that 
no arrangement of the animal or vegetable kingdom, in a straight 
line ascending like the steps of a ladder from lower to higher 
forms, can give a true idea of the relations of living beings to one 
another. These relations, on the contrary, can be expressed only 
by a ramified and complex figure which we have already compared 
to that of a genealogical tree. 
Distribution and Evolution.— Another very important depart- 
ment of biological science is that of the distribution of organized 
beings. This may be either distribution in space, geographical 
distribution: or distribution in time, paleontological distribu- 
tion. Both of these have of late years acquired increased signifi- 
cance, for we have begun to get more distinct glimpses of the laws 
by which they are controlled, of the origin of faunas and floras, 
and of the causes which regulate the sequence of hfe upon the 
_ Time, however, will not allow me to enter upon this sub- 
ject as fully as its interest and importance would enbo and a 
