XIII DIVERGENCE OF CHARACTER 145 



of a form (c) consisting of two zooids borne on a forked 

 stem and resembling Z. nutans. If, in some of the descend- 

 ants of c, this process were repeated, each of the two zooids 

 again dividing into two fixed individuals and the division 

 as before affecting the stem, we should get a species (d) con- 

 sisting of four zooids on a dichotomous stem, like Z. affine. 

 Let the same process continue from generation to genera- 

 tion, the colony becoming more and more complex ; we 

 should finally arrive at a species E, consisting of numerous 

 zooids on a complicated dichotomously branching stem, 

 and therefore resembling Z. dichotomum. 



Let us further suppose that, in some of the descendants 

 of our hypothetical form B, repeated binary fission took 

 place without affecting the stem : the result would be a new 

 form F, consisting of numerous zooids springing in a cluster 

 from the end of the undivided stem, after the manner of 

 Z. simplex. From this a more complicated umbellate form 

 (g), like Z. arbuscula, may be supposed to have originated, 

 and again starting from b with a different mode of branch- 

 ing a monopodial form (h) might have arisen. 



Finally, let it be assumed that while some of the descend- 

 ants of the forms c, d, and f became modified into more 

 and more complex species, others survived to the present 

 time with comparatively little change, forming the existing 

 species nutans, affine, and simplex : and that, in the similarly 

 surviving representatives of E, G, and H, a differentiation of 

 the individual zooids took place resulting in the evolution of 

 the dimorphic species dichotomum, arbuscula, and alternans. 



It will be seen that, on this hypothesis, the relative like- 

 ness and unlikeness of the species of Zoothamnium are 

 explained as the result of their descent with greater or less 

 modification or divergence of character from the ancestral 

 form A : and that we get an arrangentent or classification 



L 



