PROGRESS SHOWN IN EVOLUTION 



get transferred from one host to another. We thus find that 

 in respect to reproduction and life-cycle, parasites are usually 

 much more elaborate than tree-living animals; for instance, 

 the dog tapeworm can be transmitted to a dog only by 

 entering the body of an animal like a rabbit, there going 

 through a special cycle of life, and then being eaten again 

 by a dog. 



We usually say that parasites are degenerate, because we 

 note their striking loss of organs and faculties, but they are 

 only particular examples of specialization, with, as usual, 

 elaboration and improvement in one direction and loss in 

 others. The whale, in gaining blubber and tail fin, has lost 

 hair and hind limb ; the horse, in improving its middle digit 

 to a hoof, has lost the other four digits on each foot. 



But besides such one-sided specialization, there are exam- 

 ples of evolutionary improvement which are all-round, or 

 balanced, and do not deprive their possessors of their pre- 

 cious plasticity. For instance, the change from cold-blood- 

 edness to warm-bloodedness in vertebrates was such a 

 change. In becoming warm-blooded, the bird or the mam- 

 mal lost nothing which their reptilian ancestors possessed; 

 they merely acquired a new and valuable piece of vital 

 machinery, which enables them to be much more independ- 

 ent of the temperature of the outer world than they were 

 before. In the same way, the reproductive methods of rep- 

 tiles and birds represent a pure gain when compared with 

 those of their fish-like and amphibian ancestors. The evo- 

 lution of the protective membrane or amnion, which makes 

 a water cushion round the embryo, and the other embry- 

 onic membrane or allantoh, which enables the embryo to 

 breathe within the egg-shell, made it possible for reptiles to 

 be independent of water for their breeding, and so helped 

 to open up to them vast tracts of the earth's surface which 



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