25 2 
LIFE, IN ITS HIGHER FORMS. 
membering facts. There is but one division which exists 
in nature, — that of Species. Each Species is separated 
from eveiy other Species by an impassable boundary 
(whether we can in all cases determine it practically or 
not). It was originally created distinct, and distinct it 
remains. But the group of Species which we call a Genus 
is a merely arbitrary collocation ; convenient, indeed, as we 
before said, and to a certain extent natural, inasmuch as it 
is a formula for expressing the community of certain cha- 
racters ; but still arbitrary, inasmuch as it might be made 
more or less extensive, according to the pleasure of the 
naturalist who chooses the characters on which it is made 
to rest. And so of all the higher groups. 
The great Division of animal existences which we pro- 
pose now to consider presents peculiarities of structure 
and function, which we can seize and identify with great 
precision when we look at it as a whole. But if we exa- 
mine the points of contact between it and the great groups 
we have dismissed, wo find these broadly-marked distinc- 
tions becoming evanescent, and melting into those of the 
conterminous phalanx. 
One grand distinction of the higher animals is com- 
memorated in the title by which they are generally 
known, — Vertebrata. They possess an internal slceleton 
composed of many pieces, and formed of a substance which 
is not deposited, layer by layer, like the shells of Mol- 
lusca, but is capable of growth in the manner of fleshy 
tissues, being permeated both by blood-vessels and nerves, 
and undergoing a perpetual change in its component atoms. 
In its simplest form this substance is flexible and elastic, 
and is called cartilage; but by the addition, in various 
