24 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Amoeba in this particular, that, for the whole period 

 of their lives, all the functions of the organism are 

 performed by a single cell ; and, even where cells 

 remain collected into a colony, each individual member 

 of that colony performs all its own duties, and affords 

 no assistance to the rest ; there is no division of 

 labour. 



In the higher animals a very different phenomenon 

 is seen; here again the whole organism is, indeed, 

 composed of cells or cell-derivates ; but, howsoever com- 

 plex it may become, it starts always on the cycle of 

 its existence under the form of a single cell. This cell, 

 which is known as the ovum or egg-cell, undergoes a 

 series of divisions by means of which, two, four, eight 

 .... cells are produced, and these become arranged in 

 definite fashion, and take on more or less well-defined 

 functions. Here, then, different parts of the organism 

 have different duties, or, in other words, there is 

 division of labour. 



The first or lower group of organisms are asso- 

 ciated together as the Protozoa; the second, or 

 those that come after them, form the division of the 

 JVIetazoa. Did we desire to use less objective terms, 

 we might adopt for these groups the corresponding 

 terms of Cytozoa and Histozoa (Maupas), which 

 conveniently direct attention to the essential differ- 

 ence in the cells of the protozoan, and the tissues 

 of the metazoan organism. 



In attempting to arrange either of these divisions, 

 we are met at once by the fact that the changes which 

 have taken place in organisms have been in two lines 

 or directions ; there has been progress, and there 

 has been degeneration. The former we shall find 

 to be more intimately associated with a free and active 

 life, and a ready power of adaptation to changed cir- 

 cumstances ; the latter to a fixed and often to a para- 

 sitic mode of existence. 



