EVOLUTION 361 



and there are successive changes in the same direction in the chromatin 

 of the germ cells, and if these changes produce, in the adult, modifications 

 which are successively in the same direction, evolution proceeds as if 

 aimed at a definite goal. Cases in which successive steps in evolution 

 appear to have been taken in the same direction are known. Such 

 apparently directed evolution has been given the name orthogenesis; 

 or, literally, development in a straight Une. In orthogenesis the guidance 

 of the evolution is held to come from within the organism, not from the 

 environment. A case often cited in illustration is the development of the 

 horse from a five-toed ancestor, through four-toed and three-toed ances- 

 tors, to the modern condition. Some paleontologists regard this particu- 

 lar case of straight-line development in the horse as being directed by 

 the environment; but the view that it was internally guided is quite as 

 tenable. 



Orthogenesis is not to be assumed, however, in every case in which 

 there is a graded series of forms. It must be known that these forms 

 followed one another in proper order, from one extreme of the series to 

 the other. A modern example will show how one might be mistaken in 

 assuming orthogenesis where random mutation had really taken place. 

 In the fruitfty, a large number of eye colors have originated in recent 

 years. They range from a deep red to white, with the differences be- 

 tween some of the colors very sHght. Some of the intermediate colors 

 have been named cherry, pink, vermilion, eosin, buff, and tinged. If 

 these colors be arranged in a graded series with red (the normal eye color 

 of the wild fly) at one end and white at the other, they might be supposed 

 to represent the successive steps in orthogenetic evolution, with white 

 the final stage. This was not the order in which they arose, however, as 

 white was one of the first to appear. Orthogenesis must not be accepted 

 as demonstrated in a specific case, therefore, until the order of appear- 

 ance of the various modifications is known, and until it is established 

 that each member of the series was actually the ancestor of that which 

 follows it. The case of toes in the horse lacks something of proof, as 

 a case of orthogenesis, when judged by these criteria. The fossils show 

 that, in general, the three-toed horses succeeded the five- and four-toed 

 horses and were succeeded by the one-toed horse. It can only be inferred, 

 however, that the three-toed horse descended from the four-toed, and 

 gave rise to the one-toed. If this inference is correct, as it almost cer- 

 tainly is, there is only one other requirement. If the order in which the 

 changes occurred was due to factors within the animals, and was not 

 im,pressed by the environment, the gradual reduction of the toes of the 

 horse from five to one is a genuine case of orthogenesis. Regarding the 

 causation of these modifications opinions differ. Many biologists are of 

 the opinion that they arose from internal agencies, independently of the 

 environment. 



