LIFE, IN ITS HIGHER FORMS. 



membering facts. There is but one division which exists 

 in nature, — that of Species. Each Species is separated 

 from every 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, we 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, — Yertebrata. They possess an internal skeleton 

 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 



