418 GENERAL ZOOLOGY. 



are higher and more differentiated than their ancestors, 

 but occasionally and usually the result of parasitism- 

 degeneration occurs and organs not needed for the para- 

 sitic life gradually disappear. Almost all phases of this 

 can be seen in the parasitic Crustacea (p. 222) from forms 

 where the degeneration has just begun to those in which, 

 in the adult female, every crustacean feature has disap- 

 peared and the position of the species in the class is only 

 recognized from the young. In the cestode worms (p. 181) 

 degeneration has gone even further and the ancestral 

 features have been lost from the development. 



Carrying these ideas of evolution to their logical con- 

 clusion, it follows that all the millions of species now on 

 the earth, or which have lived on it in the past, have 

 arisen from a few original forms, and that these ancestors 

 must have been very primitive in their structure. The 

 farther back the divergence between two forms took 

 place the wider are the groups apart, while those more 

 closely allied must have separated in more recent times. 

 It also follows that our systems of classification should aim 

 to express the lines and degrees of blood-relationship, like 

 a genealogical tree. A third conclusion is that the totality 

 of organization is an adaptation to the surroundings, and 

 hence not infrequently similarities between two species 

 in certain matters of detail are to be explained, not as 

 inheritances from a common ancestor, but as having 

 arisen independently in response to external conditions. 



The theory of evolution explains many things otherwise 

 inexplicable. It tells us why there has been a regular 

 succession of animals in the past, as revealed to us in the 

 rocks, where we find a gradual progress from the simple 

 to the complex. It explains the peculiarities of geo- 

 graphical distribution when taken in connection with 



