CHAPTER XXVII 



THE THEORY OF EVOLUTION 



It is likely that no student of modern biology has gone far before 

 he has realized something of a progressive relationship between the 

 various groups of animals in the animal kingdom. Phylogeny refers 

 to the background of what has gone before in producing a race or 

 phylum and incorporates the thought that different species have 

 arisen from common ancestors. The general idea of it, as now ac- 

 cepted by most biologists, is that all living organisms have been 

 derived through normal reproduction and variation by adaptation 

 from simpler, more generalized ancestors. Phylogenetic relation- 

 ships of organisms and the origin of species have long been topics 

 of exceptional interest to biologists. Two questions which have 

 intrigued the thought of many are : first, whence came living ma- 

 terial? and second, what has been its course of events since the 

 time of origin? The theory of organic evolution provides the his- 

 tory of living things as interpreted by the biologist. 



The observations and thought of Charles Darwin gave the first 

 substantial support to the idea of all modern species originating 

 from preexisting organisms. Results of his careful studies were 

 presented in 1859 in his book The Origin of Species, which is now 

 a classic in the field. There is still lack of agreement among scien- 

 tists concerning the details of origin of life and species, but there 

 seems to be little doubt that species, genera, and even larger groups 

 have progressively developed since life originated on earth. The 

 ideas of continuity, development, and differentiation of living organ- 

 isms are quite generally accepted by those who have studied these 

 problems. The lines of kinship of animal groups are traced only 

 through common ancestry, and it is not ordinarily assumed that 

 direct relationship exists between representatives of modern groups. 



Sources of evidences of relationships are quite widespread, coming 

 from such studies as zoogeography, comparative anatomy, embry- 

 ology, physiology, paleontology, and even others. By combination 

 of information gained from such sources much has been learned of 

 the past history of many groups of organisms. Even these available 



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