144 SPECIES AND THEIR ORIGIN 



form A. And that we get an arrangement or classification, 

 in the form of a genealogical tree, which on the hypothesis 

 is a strictly natural one, since it shows accurately the 

 relationship of the various species to one another and to 

 the parent stock. So that, on the theory of evolution, a 

 natural classification of any given group of allied organisms 

 is simply a genealogical tree, or as it is usually called, a 

 phytogeny. 



It must not be forgotten that the forms A, B, c, D, E, F, G, 

 and H are purely hypothetical : their existence has been 

 assumed in order to illustrate the doctrine of descent by a 

 concrete example. The only way in which we could be 

 perfectly sure of an absolutely natural classification of the 

 species of Zoothamnium would be by obtaining specimens 

 as far back as the distant period when the genus first came 

 into existence ; and this is out of the question since minute 

 soft-bodied organisms like these have no chance of being 

 preserved in the fossil state. 



It will be seen that the theory of evolution has the ad- 

 vantage over that of creation in offering a reasonable 

 explanation of certain facts. First of all the varying degrees 

 of likeness and unlikeness of the species are explained by 

 their having branched off from one another at various 

 periods : for instance, the greater similarity of structure 

 between Z. affine and Z. dichotomum than between either of 

 them and any other species is due to these two species 

 having a common ancestor in D, whereas to connect either 

 of them, say with Z. arbuscula, we have to go back to B. 

 Then again the fact that all the species, however complex in 

 their fully developed state, begin life as a simple zooid which 

 by repeated branching gradually attains the adult complexity, 

 is due to the repetition by each organism, in the course 

 of its single life, of the series of changes passed through by 



