296 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



of any character that will not have been detected. The only 

 outstanding difficulty is the fact that large changes occur as 

 well as small ones ; this seems perhaps due to the fact that we are 

 witnessing the disintegration of highly developed apparatus in 

 place of its building up. 



In all this, except the last point, the work on Drosophila is 

 in agreement with my own observation of gradual variation in 

 Difflugia; with Castle's similar results on the rat; and with the 

 conclusions of paleontologists as to the gradual development of 

 the characteristics of organisms in past ages. 



But there is one point in the paleontological conclusions, as 

 set forth in the recent papers by Osborn, which is not in agree- 

 ment with the experimental and observational results on exist- 

 ing organisms; this I wish to notice briefly. Osborn sets forth 

 that in following given stocks from earlier to later ages, characters 

 arise from minutest beginnings, and pass by continuous gradations 

 to the highly developed condition. This seems in agreement with 

 experimental results, as I have tried here to set them forth. 

 Further, according to Osborn, these developing characters do 

 not show random variations in all directions, but follow a definite 

 course, which might seem to have been in some way predeter- 

 mined. And this is emphasized by the fact that the same sorts 

 of characters (horns,' for example) may arise independently, at 

 different ages, in diverse branches of the same stock, and each 

 follow in later ages the same definite course of development. 



It would appear therefore from this that there must be some 

 directing tendency, some inner necessity which drives a develop- 

 ing organ to follow a definite course. Evolution is characterized 

 by Orthogenesis, as this phenomenon has sometimes been called. 



Now it appears to me that we do not observe this in the present 

 day experimental work; by selection we can move in more than 

 one direction. I do not mean that the possible variations are 

 not limited by the constitution of the varying organism; they 

 certainly are. But there is no indication, so far as I can see, 

 that the variations push in one determinate direction only. 



Now, examining the paleontological summaries further as 

 regards this (I refer to Osborn's papers), we find certain points 



