ADAPTATIONS 205 



exopodites of the walking legs of the lobster, may have dis- 

 appeared entirely, and even the embryological evidence may 

 have been eliminated in the rapidity of development of the 

 individual; finally, new structures may have appeared without 

 recognizable homologies in the primitive forms. All of these 

 possibilities offer problems for the student of animal descent, 

 and their solution is at first hypothetical, such hypotheses being 

 based upon different biological evidence, sometimes upon the 

 facts of comparative anatomy, sometimes upon transient em- 

 bryological structures, sometimes upon geographical distribu- 

 tion, but usually upon two or more of these lines of evidence 

 taken together. The value of such hypotheses of the origin of 

 different types of animals depends upon the amount and the 

 nature of such evidence, which may be so convincing as to make 

 the hypothesis an established truth. This is, indeed, the case 

 with the theory of evolution itself, the evidence upon all sides 

 being so conclusive that no other general hypothesis of the 

 origin of the present day living beings is tenable. 



It is within the memory of many men still living, that the 

 different species of Crustacea comprising the orders mentioned 

 above would have been interpreted by intelligent people as 

 especially created types of animals having no genetic relation- 

 ship. Today, such organisms are universally believed, by 

 people who have the right of opinion, to be blood relations and 

 to have had a common ancestry from generalized Crustacea. 



B. EVOLUTION 



The change in point of view was due to the acceptance of the 

 doctrine of evolution which, in its modern form, had its start 

 with the publication of Charles Darwin's book on The Origin 

 of Species in 1859. This volume embodied the observations of 

 a keen naturalist, running over a period of thirty years, with an 

 argument for evolution, through natural selection in the struggle 

 for existence, of all kinds of living things. 



The effect of this one book, and of the acrimonious controversy 

 which it brought about, was a revolution in human thought. 



