144 



SPECIES AND THEIR ORIGIN 



there existed a Vorticella-like organism which we will call 

 A (Fig. 29), having the general characters of a single, 

 stalked zooid of Zoothamnium (compare Fig. 26, f^), and 

 suppose that, of the numerous descendants of this form, 

 represented by the lines diverging from a, there were some 

 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 



DIMORPHIC 

 HOUOMORPHIC 



Fig. 29. — Diagram illustrating the origin of the species oi 

 Zoothamnium by evolution. 



not for our present purpose how it may have been 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 

 distal end of the stalk : this would result in the production 



