102 RELATIONS OF MAN WITH EXTERNAL NATURE. 



are functions performed by the same set of vessels. In like manner, 

 the function of circulation itself becomes more and more simplified, 

 and finally disappears in animals which are destitute of vessels con- 

 taining a common nutritive fluid. If he turns to digestion, he dis- 

 covers some animals destitute of digestive glands, others possessing no 

 distinct alimentary canal separate from the general cavity of their 

 body; and lastly, others which are even destitute of a body-cavity. 

 In the last case, nutrition is accomplished by the direct application of 

 the surface of the animal to its food, and by the simple process of 

 direct absorption into its substance. Viewed in this manner, the 

 function of digestion is seen to be reduced to the phenomena of solu- 

 tion and absorption ; whilst together with absorption, sanguification 

 and circulation, which are subsidiary functions, it resolves itself into 

 one common function, viz., nutrition. This, indeed, is the simplest 

 expression of all the nutritive functions, arid is fundamentally repre- 

 sented by the conjoined assimilative and plastic vital property of the 

 simplest organized tissue, or the simplest form of independent animal 

 life. Lastly, if we trace back the secretive function, we find that a 

 complex organ, like the liver of man and the higher animals, is, in the 

 lower ones, represented by a cluster of follicles, by a single follicle, or 

 by a group of nucleated cells upon the surface of a membrane ; and 

 hence we perceive that the essential character of the function of secre- 

 tion, consists in a modification of the same common property of nutri- 

 tion, which is named nutritive secretion ; and so on of other functions. 

 To trace these points of comparison, both of structure and of func- 

 tion, between man and animals, frequent reference will have hereafter 

 to be made to facts and details, which will be easier of comprehension, 

 if we first take a general view of the animal kingdom. Motives of 

 utility, and want of space, necessitate the selection and employment 

 of one system of classification only ; and preference is here given to 

 the arrangement proposed by Professor Huxley in his Elements of 

 Comparative Anatomy, which, besides containing original suggestions, 

 incorporates the improvements of the modern German school. 



Outline* of the Animal Kingdom. 



The dependencs of the classification of animals on their internal structure 

 is fully illustrated in the Cuvierian system, which forms the foundation of all 

 modern arrangements ; it has, however, undergone modification, through the 

 further application of the anatomical method, and more extended inquiries 

 into the structure of many of the lower animals, which from want of means 

 or of opportunity of investigation, were but imperfectly known to Cuvier. In 

 his great contribution to comparative anatomy and zoology (Le Regne Animal) 

 he divided the entire animal kingdom into four subdivisions, named subkiny- 

 doms. These subkingdoms were composed of nineteen primary subdivisions 

 named classes, which were further broken up into seventy-seven orders, and 

 these again into further groups, ultimately separated into genera and species. 



