A DEFINITION OF EVOLUTION 



resenting the first and fifth toes ) . Yet ah-eady the third toe was more 

 prominent, both front and rear, than the others ( Figure 25 ) . 



The line of descent appears to pass, witli moderate changes at each 

 step, from Hyracothcriuni through Mesohippiis, Miohippiis, Parahippus, 

 Mert/chippus, and PUohippus to Equus, the horses of today and their al- 

 lies. The changes achieved are great indeed. The size has increased from 

 that of a house cat to that of the Percheron. The cheek teeth, originally 

 low-crowned and rather primitive, have become very high-crowned and 

 highly specialized for grinding coarse, siliceous grasses. These changes 

 have been associated with elongation of the jaws and related parts of the 

 head. The evolution of siliceous range grasses occurred simultaneously, 

 and probably served as a selective force acting upon the horses. The limbs 

 were elongated, and simplified by the loss of some parts (toes) and the 

 fusion of others (metacarpals and metatarsals). The neck became much 

 longer and more mobile. 



This phylogeny, emphasizing as it does the six extinct genera believed 

 to be in or near the direct line leading to living horses, should not be 

 interpreted as meaning that horse evolution has gone in a definite direc- 

 tion from the beginning, as though horses had from the first known where 

 they were going and had taken the most direct steps to get there. It must 

 be remembered that the record includes thirteen other genera of the 

 family Equidae, as well as the eight genera of the family Paleotheriidae. 

 These many genera varied in several directions, sharing some character- 

 istics with their cousins in the "main line," but differing from them in 

 many others. Some lines of descent, for example, did not develop high- 

 crowned teeth. At every period in the evolution of the horse there have 

 been widely varying genera and species. Natural selection has simply 

 eliminated the great majority of these. 



The validitv of this series of horse fossils has been conceded bv everv- 

 one, but some opponents of evolution have contended that the mere fact 

 that such a series of genera existed in a succession from the most primitive 

 to the most specialized does not require that the latter be descended from 

 the former. They argue that each may have been independently created, 

 and that there is no reason why they should not have been created in an 

 orderly sequence. There is no answer to this, simply because it waives 

 the evidence altogether. One could just as well say that there is no reason 

 why they should have been created in any particular sequence. It seems 

 far more probable that the appearance of descent is there because the 

 specialized horses actually are descended from the primitive ones. But 

 when one considers also the innumerable examples from all major groups 

 of organisms which parallel the above case, the probabilit\' that this ap- 

 pearance of descent is misleading becomes very remote. 



EVIDENCE FROM GENETICS 



The final line of evidence for (^volution is drawn from genetics, the science 

 of heredity. It is not intended to review this subject here, for the bearing 

 of genetics on evolution will make up a large part of the succeeding chap- 



74 



