EVIDENCES OF ORGANIC EVOLUTION 31 



3. MORPHOLOGY. On comparing together the dif- 

 ferent members of one of the great groups or classes 

 of animals or plants, we find the same fundamental 

 plan of organization running through all of them. 

 Series of corresponding organs are often to be made 

 out which are built upon the same general scheme, 

 although their functions may be quite dissimilar ; so 

 that, for instance, in the hand of a man, the paw of a 

 dog, the wing of a bat, and the paddle of a whale, 

 almost identically the same series of bones can be 

 traced. An obvious explanation is to be found in the 

 supposition that these parts have arisen by the 

 divergent modification of parts which were originally 

 identical. 



4. GEOGRAPHICAL DISTRIBUTION. Observation 

 shows that groups of closely allied creatures are often 

 found living in neighbouring districts, and that when 

 such a barrier as an ocean or a range of lofty moun- 

 tains is passed an entirely new fauna and flora are 

 usually to be met with. These facts may be explained 

 by the hypothesis that allied groups of species origi- 

 nated by a process of descent in the same countries 

 which they now inhabit, and they can be explained 

 by no other known hypothesis. 



5. THE GEOLOGICAL SUCCESSION OF ORGANISMS. 

 The general facts regarding the distribution of allied 

 species of animals and plants in time point in pre- 

 cisely the same direction as those relating to their 

 distribution in space. In a few cases, notably in that 

 of the extinct horse of North America, a long chain of 

 possibly ancestral types has been found leading back 



