140 SPECIES AND THEIR ORIGIN LESS. 



species we understand an assemblage of individual or- 

 ganisms, whether simple or compound, which agree with one 

 another in all but unessential points, such as the precise 

 number of zooids in Zoothamnium, which may vary con- 

 siderably in the same species, and come, therefore, within 

 the limits of individual variation. Similarly, what we mean 

 by a genus is a group of species agreeing with one another 

 in the broad features of their organization, but differing in 

 detail, the differences being constant. 



A comparison of the six species described brings out 

 several interesting relations between them. For instance, it 

 is clear that Z. arbuscula and Z. alternans are far more 

 complex /.<?., exhibit greater differentiation of the entire 

 colony, than Z. simplex, or Z. nutans ; so that, within the 

 limits of the one genus, we have comparatively low or 

 generalized, and comparatively high or specialized species. 

 Nevertheless, a little consideration will show that we cannot 

 arrange the species in a single series, beginning with the 

 lowest and ending with the highest, for, although we should 

 have no hesitation in placing Z. nutans at the bottom of 

 such a list, it would be impossible to say whether Z. affine 

 was higher or lower than Z. simplex, or Z. arbuscula than 

 Z. alternans. 



It is, however, easy to arrange the species into groups 

 according to some definite system. For instance, if we take 

 the mode of branching as a criterion, Z. nutans, afrlne, and 

 dichotomum will all be placed together as being dichoto- 

 mous, and Z. simplex and arbuscula as being umbellate 

 the zooids of the one and the branches of the other all 

 springing together from the top of the main stem : on this 

 system Z. alternans will stand alone on account of its mono- 

 podial branching. Or, we may make two groups, one of 

 dimorphic forms, including Z. arbuscula, alternans, and 



