IT 
We have still been using these conventional terms, “ high,” 
“low,” “nobler,” “more perfect,” &c., to indicate that progres- 
sive development of organic life, without deciding what principle 
shall guide us in their application. This is really the most 
important part of our subject. All conceptions of progress are 
based on it. We have said that there is no linear succession of 
advancing forms, that is to say, they cannot be truly depicted in 
a straight line. But even if we look at the plan of nature under 
the image of a tree that will not help us in our present quest. 
We cannot conceive of a tree without giving to it some altitude 
as a whole, and generally picture it with branches of varying 
height. 
The branches of our imaginary Tree of Life are made up of 
different families or genera each with numerous species. How is 
the place of each species that helps to make the branch decided ? 
Which is to be put above, which is below another? What is the 
measure of value ? 
There are two lines of inquiry which commend themselves to 
the systematist,—function and structure, 
Seeing that estimation of function is beyond us, let us try 
how far classification by structure will help us in the solution of 
the problem. 
Let us take function first, and in doing so consider man as the 
head of creation, and to be such by virtue of his intelligence, or, 
more broadly, by his nervous and cerebral activity. If, therefore, we 
take this, as Lamarck did, as the basis of classification, and if we 
could arrange all animal life in a descending scale from man, then 
we should have a measure of general application to all its 
forms. But this is impossible. There is no measure of such 
manifestations of vitality. Besides, it totally. excludes the 
vegetable kingdom. There is no nervous activity here to 
measure. Plants are accumulators of power, not expenders. 
Structure is, to a large extent, an index of function. 
Complex structure implies varied functions. Knowing the 
structure of an organism which can, to a great extent, infer its 
functions. 
There is a large class of animals of infinite variety of colour, 
size, and outward form all possessing a backbone. Moreover, 
there are many structural details which are always associated with 
this backbone which terminates in a skull. The digestive system 
is on one side, the nervous on the other. The chain of vertebrz 
enclose a spinal cord, the muscles are attached to an osseous 
frame-work, the limbs are variously modified, &c. Thus the 
Vertebrates, we say, form a great natural class. 
Looking again at a vast number of other creatures, we find 
them built up on quite a different plan. There is no backbone, 
the so-called skeleton is simply a hardened skin, the limbs 
are hollow tubes leading from the cavity of the body. The 
