50 ' DOCTRINE OF EVOLUTION 



tacea, but they have lost their eyes as well as some 

 other structures that are most useful in animals with 

 a free existence, because they have adopted a fixed 

 mode of life, which has also brought about the loss 

 of the original freely jointed character of the body. 

 A tapeworm as an example of internal parasites is 

 an extremely degenerate form which lacks a digestive 

 tract, because this is superfluous in an animal which 

 lives bathed in the nutrient fluids of its host. Compar- 

 ing it in other respects with other low wormlike crea- 

 tures, it appears to be a relative of pecuhar simple worms 

 with complete organization and independence of life. 

 All these degenerate forms enlarge our conception of 

 adaptation by adding the essential point that progress 

 is not always the result of evolution. Indeed we have 

 learned this in the case of vestigial and rudimentary 

 structures of higher forms like whales, and now we find 

 that entire animals may degenerate as a result of 

 changes no less adaptive than progressive modifications. 

 Passing by other invertebrate groups made up of 

 species arranged like higher animals in smaller and 

 larger branches according to their degree of fundamental 

 similarity, we arrive at a place in the scale occupied 

 by two-layer animals without the highly developed 

 and clearly differentiated organic systems of the forms 

 above. The fresh-water animal Hydra exemplifies 

 the creatures of this level, where also we find sea- 

 anemones and the soft polyps which form corals and 

 coral reefs by their combined skeletons. Hydra is an 

 animal to which we must return again and again as we 

 study one or another aspect of organic evolution. In 

 general form it is a hollow cylinder closed at one end, 



