658 



ORGANIC EVOLUTION 



This animal also possessed teeth better 

 adapted for chewing grass and its general 

 anatomy resembled the modern horse. 



Horses that appeared in the Pleistocene 

 had acquired essentially the same propor- 

 tions as the modern horse. There is a sin- 

 gle hoof on each foot, although paired 

 lateral splints give evidence of the ancient 

 toes of its forebears. On rare occasions a 

 colt is born with the two lateral toes vis- 

 ible externally, thus reminiscent of its an- 

 cestors. The tendency toward a longer face 

 has continued with the result that the eye 

 is located at the edge of the most posterior 

 molars. The teeth are beautifully adapted 

 for grinding grasses, which form the prin- 

 cipal diet of these animals. The size is 

 larger than any of the earlier horses, and 

 by selective breeding man has produced 

 the kinds of horses he desires, from Shet- 

 land ponies to draft horses. 



Through a period of 60 million years, 

 the horse has evolved from a tiny animal 

 living in the forest for protection and feed- 

 ing on succulent leaves to a huge grass- 

 eating beast roaming the prairies where 

 fleet-footedness is essential for survival. 

 This seems to be a case of straight-line 

 evolution, almost as if the horse "wanted" 

 to change its way of life from a shy, seclu- 

 sive forest dweller to a swift-running prairie 

 animal. Without going into a lengthy dis- 

 cussion of the reasons, it is now becoming 

 clear that the evolution of the horse has not 

 been as straight-line as might appear. Simp- 

 son, for example, believes that horses as 

 well as other animals have never gone 

 straight toward a goal but, instead, have 

 pushed out in many directions at succeeding 

 stages in their evolution, with one part of 

 the anatomy such as the teeth going for- 

 ward more rapidly at times than other 

 parts. In other words, there have been 

 many lines of horse evolution all along the 

 way, each evolving in different directions 

 but in the general direction of the modern 

 horse. Some of these survived to give rise 

 to the next great step whereas others be- 



came extinct. Indeed, he believes that the 

 most direct line from Eohipptis was not to 

 the modern horse but to an extinct horse 

 called Hypohippus. Apparently, then, the 

 direction from Eohipptis to Eqinis has not 

 been a straight one but rather one with 

 many deviations, jogs left and right, some 

 paths leading to extinction, others carrying 

 on to the next, perhaps only then to become 

 extinct. 



What, then, keeps this whole trend de- 

 limited to one general progressive course? 

 There must be forces operating continu- 

 ously to bring about this over-all trend, 

 forces that seem forever to "nudge" the line 

 in a general direction. Environment alone 

 cannot be the answer because we know 

 from genetics that mutations occur regard- 

 less of environmental changes, although it 

 undoubtedly does play a part. When neither 

 of two factors is the whole answer, both 

 must operate simultaneously; both the en- 

 vironment and the functional capacities 

 of the organism interact, resulting in the 

 ultimate fitting of the latter to the former. 

 This we call adaptation, the most signifi- 

 cant outcome of evolution, about which we 

 shall have more to say later. 



Opportunity in evolution 



Another feature that is probably impor- 

 tant in oriented evolution, which also in- 

 volves the relation of organisms and envi- 

 ronment, is the relative time or coincidence 

 with which these impinge upon one another. 

 When the environment is just right for an 

 animal to evolve in a certain direction, the 

 animal must not only be present but must 

 also be sufficiently adaptable (have suffi- 

 cient mutations available) to take advan- 

 tage of the opportunity. Simpson called this 

 "Opportunism of Evolution." Because of 

 these environmental opportunities, animals 

 follow within certain restricted limits, pro- 

 gressing toward an apparent goal, as in 

 the case of the horse. As the environment 

 changes, the organisms change with it. Liv- 

 ing things themselves constitute more op- 



