MORPHOLOGICAL DIFFERENTIATION. 47 



the most characteristic physiological peculiarity of the plant 

 is its power of manufacturing protein from chemical com- 

 pounds of a less complex nature. 



The most characteristic morphological peculiarity of the 

 animal is the absence of any such cellulose investment. 1 The 

 most characteristic physiological peculiarity of the animal is 

 its want of power to manufacture protein out of simpler 

 compounds. 



The great majority of living things are at once referable 

 to one of the two categories thus defined ; but there are some 

 in which the presence of one or other characteristic mark 

 cannot be ascertained, and others which appear at different 

 periods of their existence to belong to different categories. 



II. THE MOEPHOLOGICAL DIFFERENTIATION OF ANIMALS. 



The simplest form of animal life imaginable would be a 

 protoplasmic body, devoid of motility, maintaining itself by 

 the ingestion of such proteinaceous, fatty, amyloid, and min- 

 eral matters as might be brought into contact with it by ex- 

 ternal agencies ; and increasing by simple extension of its 

 mass. But no animal of this degree of simplicity is known 

 to exist. The very humblest animals with which we are ac- 

 quainted exhibit contractility, and not only increase in size, 

 but, as they grow, divide, and thus undergo multiplication. 

 In the simplest known animals — the Protozoa — the proto- 

 plasmic substance of the body does not become differentiated 

 into discrete nucleated masses cr cells, which by their meta- 

 morphosis give rise to the different tissues of which the adult 

 body is composed. And, in the lowest of the Protozoa^ the 

 body has neither a constant form nor any further distinction 

 of parts than a greater density of the peripheral, as com- 

 pared with the central, part of the protoplasm. The first 

 steps in complication are the appearance of one or more 

 rhythmically contractile vacuoles, such as are found in some 

 of the lower plants ; and the segregation of part of the in- 



1 No analysis of the substance composing the cysts in which so many of the 

 Protozoa inclose themselves temporarily has yet been made. But it is not im- 

 probable that it may be analogous to chitin ; and, if so, it is worthy of remark 

 that, though chitin is a nitrogenous body, it readily yields a substance appar- 

 ently identical with cellulose when heated with the double hyposulphite of 

 copper and ammonia. It is possible, therefore, that the difference between 

 the chitinous investment of an animal and the cellulose investment of a plant 

 may depend upon the proportion of nitrogenous matter which is present in 

 each case in addition to the chitin. 



