EVOLUTION 357 



changes must be kept distinct from the question whether the environ- 

 ment may cause evolution. The latter question involves a smaller burden 

 of proof than does the former. 



Nothing here stated about the possible effect of environment, how- 

 ever, is in any way contradictor^' to the preceding paragraph about muta- 

 tions and the chromosomes. For even if environment does produce 

 heritable changes in organisms it is practically certain that the first 

 effect wrought by the environment is a change in the chromatin, or 

 whatever determines development and heredity. One could subscribe 

 to the view that changes in the chromatin are the sole cause of mutations, 

 and still go back one step farther and admit that the environment caused 

 the alterations in the chromatin. Those who hold that environment has 

 nothing to do with evolution regard the modifications of the chromatin 

 as arising "spontaneously" from unknown causes within the animal. 



COURSE OF EVOLUTION 



Just as there has been confusion in the minds of writers and observers 

 between the fact of evolution and the cause of evolution so has there been 

 confusion between the cause of evolution and its course. It is important 

 that these two matters be kept entirely distinct. One may discuss the 

 direction in which species have become modified, without knowing or 

 even caring why they became altered. Even by some biologists, those 

 factors which guide evolution have been spoken of as if they started evo- 

 lution. It is not inconceivable that the same agency both starts and 

 directs evolution, but it need not do so. A projectile is thrown from a gun 

 by the force of the explosion; but gravity determines the curve it de- 

 cribes after leaving the muzzle. The engine and the steering-wheel of a 

 motor-car are two different things. 



The Course of Evolution Demonstrated or Surmised.— Discussion of 

 the course of evolution may involve one or both of two distinct questions. 

 One is, what course has evolution taken? The other, what directed 

 evolution in that course? The former question can be answered in part 

 with considerable accuracy. So long as limited groups of animals are 

 considered, it is often possible to determine relationships with a fair 

 degree of certainty. There can be httle doubt, for example, that the 

 horse has descended from a five-toed ancestor of small size. The fossils 

 show at least so much. They even show the order in which all toes except 

 the third were reduced. Fossil cephalopods indicate that these animals 

 changed from straight forms with straight septa to coiled forms, still with 

 fairly straight septa; and that then the septa became bent and finally 

 crinkled — in certain lines of descent. The shortening of the elephant's 

 skull, the elongation and subsequent contraction of the lower jaw, and 

 the reduction in the number of the teeth, are likewise beyond doubt. 



