118 THE THEORY OF EVOLUTION 



of the organization from the degree of perfection i.e. 

 from the greater or less strictly executed distribution of 

 the separate vital functions to special tissues, i.e. organs. 



The ' type ' determines by itself alone nothing as 

 regards the perfection of the animal ; the degree of 

 differentiation in separate tissues and organs yields, how- 

 ever, an objective criterion i.e. one based on reality 

 of the height of organization. That applies particularly 

 also to the differentiation of the nervous system. 1 

 According to the ' plan of construction ' and the height 

 of organization there is effected also at present the 

 systematization of the animals and plants into a few 

 well-defined groups. 



As a rule the zoological textbooks divide the animal 

 kingdom into seven, nine, and even more families 

 (Stamme). R. Hertwig holds to seven in his well-known 

 ' Lehrbuch der Zoologie/ E. Selenka 3 adopts ten 

 and distinguishes them as follows : 



I. Monocellular Protozoa (1) 



II. Leaf animals (Metazoa) witli cellular 

 differentiated tissues and organs: 



1. No bodily cavity as blood reser- 

 voir ; Coelenterata : 

 (a] Bodies non-symmetrical Spongise (Sponges) (2) 



1 Recently here and there the biologists have been denied all right to 

 speak of various grades of perfection or heights of organization, or, at the 

 best, they are yielded to only for practical reasons (by reason of the division 

 of matter). See B. Franz in Biol. Zentralblatt, 1911, p. 1 : ' What is a 

 higher organism ? ' We shall refer again to this article. 



2 Zoologisches Taschenbuch, I, Lepizig, 1897, p. 1. For quick information 

 the two volumes of this handbook are much to be recommended ; they will 

 always provide a clear and short grounding in the systematic arrangement. 



