STRUCTURE AND DEVELOPMENT 53 



clearly occupy different places in the scale which begins 

 with the protozoa and extends upward to the most 

 complicated and differentiated animals. Hydra takes 

 its place above the protozoa for obvious structural 

 reasons ; worms belong to a still higher zone, surpassed 

 by the more complex jointed animals like Crustacea 

 and insects. Far above these are the vertebrates, 

 among which we have already demonstrated the occur- 

 rence of different grades of organization, from the fish 

 up to the higher amphibia and reptiles, and beyond 

 in two directions to the diverging birds and mammals. 

 The basic characteristics of every group in a high 

 position may be traced back to some one or another 

 of the divisions at a lower level, so that the general 

 sequence of the structural levels from low to high 

 becomes intelligible as the order of their evolution. 



To my mind the rudimentary and vestigial structures 

 of animals are in themselves proof positive of a natural 

 history of change. The few illustrations can be re- 

 inforced by countless examples offered by every group 

 of living animals. If such structures have not evolved 

 naturally by degenerating from more efficient counter- 

 parts in ancestors of earlier times, and if they have 

 been specially created, they are utterly meaningless 

 and their very existence is unreasonable. If common 

 sense is to be employed, they demonstrate evolution. 



Everywhere throughout the whole series animals 

 place themselves in a treelike arrangement, for in their 

 respective levels they occur like leaves at the ends of 

 the lines of descent which have led up to them and 

 which are comparable to the branches and limbs 

 arising from the trunk of a tree. Thus the major 



