DESCENT WITH CHANGE 351 



the data seems to prejudge the question is the most cogent pre- 

 sumptive evidence for evolution. It is true that there are wide 

 differences of opinion among biologists in regard to the factors 

 which have brought about the evolutionary change — but there 

 are none in regard to the fact of evolution itself. It will be con- 

 venient, therefore, first to summarize a few of the evidences of evolu- 

 tion, and then to present certain modern views in regard to the 

 methods of evolution. 



A. Evidences of Organic Evolution 



To one who has thoughtfully followed the preceding pages 

 there must immediately occur many facts which are readily and 

 reasonably interpreted from the point of view of descent of one 

 species from another, but which are entirely obscure from that 

 of the special creation of species. For instance, one will recall the 

 cellular structure of all organisms; the method of origin and the 

 fate of the germ layers in animals; the interrelationship of the 

 urinary and reproductive systems in the Vertebrates; the com- 

 parative anatomy of the vascular and skeletal systems of Verte- 

 brates; the similarity of the physical basis of inheritance in 

 animals and plants: in a word — the 'unity in diversity' 

 that pervades the world of living things. (Figs. 122, 129, 

 141, 227.) 



In general, such are the types of data which support the evo- 

 lution theory. Although the evidence, from the nature of the 

 case, must be indirect, it is none the less impressive, chiefly because 

 the facts for evolution are from such diverse sources and all con- 

 verge toward the same conclusion. The theory of evolution reaches 

 the highest degree of probability, since in every branch of botany 

 and zoology all the data are most simply and reasonably explained 

 on the basis of descent with change. It is a cardinal principle of 

 science to accept the simplest conceptions which will embrace all 

 the facts. 



Assuming the reader's familiarity with the contents of this 

 volume up to the present point, it is now necessary to summarize 

 some of the most important evidence from various subdivisions of 

 biology. But, as will soon appear, it is impossible to arrange the 

 facts in natural groups because the evidence from one merges into 

 that from another — the evidence interlocks. 



