CHAPTEE XVII 



Connecting links Homology and analogy Convergent evolution 

 Change of function Vestigial structures Eeversion. 



IN presenting the evidence of organic evolution it is convenient 

 to separate that derived from the study of comparative anatomy 

 from that afforded by taxonomy, but the separation is not 

 a logical one, for classification is necessarily based upon com- 

 parative anatomy, and much of the evidence might be equally well 

 dealt with under either heading. 



The mere fact that we are able to arrange existing organisms 

 in progressive series, of which the extremes are connected by 

 intermediate forms, in itself suggests that one form has been 

 derived from another. This gradation is equally obvious whether 

 we study entire organisms or confine our attention to their com- 

 ponent parts or organs. It is illustrated very clearly by the 

 accompanying diagram of the modifications exhibited by the 

 siliceous spicules of certain sponges (Fig. 88). These micro- 

 scopic organs of the sponge, each of which arises within a single 

 cell, exhibit an astonishing diversity and beauty of form. The 

 extreme types are as different from one another as we can well 

 imagine, but in the particular group of sponges from which our 

 illustration is taken (the Tetraxonida) all may be derived from the 

 same four-rayed architype (number 1 in the centre of the 

 figure) and numerous intermediate forms mark out the lines 

 along which evolution has taken place. An analogous diagram, 

 with a triaxonid architype, could easily be constructed for those 

 remarkable deep-sea sponges known as the Triaxonida or 

 Hexactinellida, and another for the calcareous sponges. 



Amongst the higher animals also innumerable illustrations of 

 the same general principle of gradation in structure, even in co- 

 existing types, are met with, and are to be explained in the 

 way indicated in the last chapter, as expressions of phylogenetic 

 relationship. Take, for example, those now widely contrasted 

 groups of vertebrate animals the Reptilia and the Mammalia. The 



