1 86 ORGANIC EVOLUTION 



eggs and spermatozoa held together by a little soft tissue 

 which surrounds these germ cells. In this condition, appar- 

 ently to allow of its growth to still larger size, it begins to 

 protrude from the body of the crab, becoming in the end a 

 bag of considerable size held to the crab by root-like pro- 

 cesses that penetrate through the shell and into the body of 

 the crab, and take up nourishment from its blood (Plate 

 101, ). Soon the Saccnliua bursts and the eggs are set free, 

 and each starts upon a new cycle of development similar to 

 that described. 



Life under the conditions of parasitism is very easy, and 

 it is no wonder that many animals and plants have been 

 adapted to such life. Since many organs essential to the 

 welfare of self-dependent animals are useless to parasitic 

 forms, we find that parasitism is usually associated with the 

 loss of these useless organs; or, in other words, we can say 

 that parasitism results in simplification. We have quoted 

 an extreme instance of simplification. There are other 

 cases of as great simplification of structure, but in most 

 instances the degeneration is not so pronounced. Phe- 

 nomena of degeneration, however, are not observed only in 

 parasitic forms but are very general, and animals which as a 

 whole are not degenerate, usually have some of their organs 

 degenerate. In our own bodies are many such degenerate 

 organs. (Skin muscles, except over the face ; ear muscles, 

 Plate 96; tail, coccyx, Plate 91 ; third eyelid, Plate 97; hair 

 of body, Plate 93 ; vermiform appendix, Plate 96, B ; and a 

 hundred others.) Many phenomena of simplification are just 

 as much the result of natural selection as are the phenomena 

 of increasing complexity of structure. Natural selection 

 brings about more perfect adaptation to the conditions of 



