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CHAPTER XIX 



GAPS IN THE EVOLUTIONARY LINES- 

 MUTATIONS 



THE second of the three sets of facts to be 

 considered in connection with the develop- 

 ment of animal forms concerns the imperfec- 

 tions in the evolutionary picture as portrayed by a 

 branching tree-like figure. 



All the evolutionary lines are frequently interrupted 

 by gaps of various w^idths, and these gaps are often 

 very broad. Thus in the horses, as has been pointed 

 out by Dr. James W. Gidley and by Professor W. D. 

 Matthew, there is a considerable gap between the 

 Eocene and the next following (Oligocene) types, 

 and there are numerous other gaps, many of the types 

 of horses being more or less, and sometimes rather 

 widely, isolated from their nearest relatives. 



In the horses, as stated by Professor Matthew, we 

 "have in many cases a succession of collateral ances- 

 tors so nearly related to the direct genetic line as to 

 afford, when critically studied with due recognition 

 of their status, a clear record of the physical evolution 

 of the race, sometimes in more general, sometimes in 

 more detailed terms according to the nearness of their 

 approximation to the direct ancestral line." 



So we see that even in the horses the tree-like figure 

 is after all only an approximation to the truth. The 

 twigs of the tree do not actually join the branches, 



