A VIEW IN PERSPECTIVE. 139 



increase in the profusion of the higher orders of all 

 classes, and the general increase in the abundance 

 and diversity of the families of animals which we can 

 trace through all ages, shows conclusively that there 

 has been a distinct advance in complexity ef type 

 from the earliest times up to to-day. 



Although there has been thus a general advance, 

 it is certainly true that it has not affected all orders 

 of animals alike. We have already noticed that some 

 have remained practically stationary in type. Some 

 have actually gone down hill instead of up. Many 

 groups of animals have advanced to a culmination 

 and then begun to disappear ; and in the disappear- 

 ing of animals we can frequently discern evidence of 

 a reversal of the process of development, the later 

 appearing animals becoming of a distinctly lower 

 type than their relatives at the period of culmination. 

 This has been specially well shown in the ammon- 

 ites which passed out of existence in the Cretaceous 

 (8). In many orders of animals the degeneration has 

 been produced from other causes. Degeneration is 

 always certain in the animal kingdom when any 

 organ ceases to be used. Many orders of animals by 

 becoming parasitic upon others cease to have any 

 use for some of their organs. The result is a very 

 general degeneration until the structure of the ani- 

 mal as a whole has become markedly degraded. 

 Any of the orders of parasitic animals will serve as 

 illustrations of this fact, and all of the sub-kingdoms 

 show examples of such degradation. Among fami- 

 liar examples may be mentioned fleas, which have 

 lost their wings ; tape worms, which have lost all of 



