142 



SPECIES AND THEIR ORIGIN 



Suppose that at some distant period of the world's history 

 there existed a Vorticella-like organism which we will call 

 A (Fig. 30), having the general characters of a single 

 stalked zooid of Zoothamnium (compare Fig. 27, F 2 ), and 

 suppose that, of the numerous descendants of this form, 

 represented by the lines diverging from A, there were some 



Branching dichotomous 



Branching 

 umbellate 



L^ "*-i 5? 



?*%) 



t 



<a 



Branching 

 monopodial 



Z. nut an s 



Z dichoiomtim 

 Z. affine 



Z. arbusciila 



*^ 



DIMORPHIC 



\ 



HOMOMORPHfC 



FIG. 30. Diagram illustrating the origin of the species of 



Zoothamnium by evolution. 



in which both the zooids formed by the longitudinal division 

 of the body remained attached to the stalk instead of one of 

 them swimming off as in Vorticella. The result it matters 

 not for our present purpose how it is caused would be a 

 simple colonial organism consisting of two zooids attached 

 to the end of a single undivided stalk. Let us call this 

 form B. 



Next let us imagine that in some of the descendants of B, 

 represented as before by the diverging lines, the plane of 

 division was continued downwards so as to include the 



