350 



CHAPTERS ON EVOLUTION. 



theory. The stable state forces the animal to remain as it now is, or 

 as it has been in all times past ; the progressive tendency will make 

 it a more elaborate animal ; and the progress of degeneration will, on 

 the other hand, tend to simplify its structure. It requires no thought 

 to perceive that progress is a great fact of nature. The development 

 of every animal and plant shows the possibilities of nature in this 

 direction. But the bearings of degeneration and physiological back- 

 sliding are not, perchance, so clearly seen. Hence, to this latter aspect 

 of biology we may now specially direct our attention. 



That certain animals degenerate or retrogress in their develop- 

 ment before our eyes to-day, is a statement susceptible of ready and 

 familiar illustration. No better illustrations of this statement can be 

 found than those derived from the domain of parasitic existence. When 

 an animal or plant attaches itself partly or wholly to another living 

 being, and becomes more or less dependent upon the latter for 

 support and nourishment, it exhibits, as a rule, retrogression and 

 degeneration. The parasitic " guest " dependent on its "host" for 



lodging alone, or it may be for both 

 board and lodging, is in a fair way to 

 become degraded in structure, and, 

 as a rule, exhibits degradation of a 

 marked kind, where the association 

 has persisted sufficiently long. Para- 

 sitism and servile dependence act 

 very much in structural lower life as 

 analogous instances of mental de- 

 pendence on others act in ourselves. 

 The destruction of characteristic 

 individuality and the extinction of 

 personality are natural results of that 

 form of association wherein one form 

 becomes absolutely dependent on 

 another for all the conditions of life. 

 A life of mere attachment exhibits 

 FIG. ^-COMMON TAPEWORM (T*nia similar results, and organs of move- 

 soiium). i. The head extremity, mag- ment disappear by the law ofdisuse. A 



nified, showing hooks (a), and suckers Hicrpctivp C vcr<=>m ic a cnr>frflin't\7 fr an 

 (6, c); d, the neck, with immature joints. CUgCStlVe SV Stem IS a Superfluity tO 3 n 



2. A joint, largely magnified, showing animal which, like a tapeworm (Fig. 



SLj^^rf^^^S^ 249), obtains its food ready-made in 



the very kitchen, so to speak, of its 



host. Hence the lack of a digestive apparatus follows the finding of a 

 free commissariat by the parasite. Organs of sense are not necessary 

 for an attached and rooted animal ; these latter, therefore, go by the 

 board, and the nervous system itself becomes modified and altered. 

 Degradation, wholesale and complete, is the penalty the parasite has 



