300 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



To summarize then what I have obtained from experimental 

 work combined with a survey of the work of others, the impression 

 left is as follows: 



1. Experimental and observational study reveals that organ- 

 isms are composed of great numbers of diverse stocks differing 

 heritably by minute degrees. 



2. Sufficiently thorough study shows that minute heritable 

 variations — so minute as to represent practically continuous 

 gradations — occur in many organisms; some reproducing from a 

 sing'.e parent others by biparental reproduction. 



3. The same thing is reported from paleontological studies. 



4. On careful examination we find even that the same thing 

 is revealed by such mutationist work as that on Drosophila; 

 single characters exist in so many grades due to minute altera-' 

 tions in the hereditary constitution as to form a practically 

 continuous series. 



5. It is not established that heritable changes must be sudden 

 large steps; while these may occur, minute heritable changes 

 are more frequent. 



6. It is not established that heritable variations follow a 

 definite course as if predetermined; they occur in many directions. 



7. It is not established that all heritable changes are by 

 disintegration; although many such do occur, they cannot be 

 considered steps in progressive evolution from the visibly less 

 complex to the visibly more complex. 



Evolution according to the typical Darwinian scheme, through 

 the occurrence of many small variations and their guidance by 

 natural selection, is perfectly consistent with what experimental 

 and paleontological studies show us; to me it appears more 

 consistent with the data than does any other theory. 



BIBLIOGRAPHY 



Bateson, W., 1914. Address of the President of the British Association for 

 the Advancement of Science. Science, 40: 319-333. Bridges, C. B., 1916. Non- 

 disjunction as proof of the chromosome theory of heredity. Genetics, 1: 1-52; 

 107-163. Castle, W. E., 1915. Mr. Midler on the constancy of Mendelian char- 

 acters. Amer. Nat., 49: 37-42. Castle, W. E., 1915 a. Some experiments in 

 mass selection. Amer. Nat., 49: 713-726. Castle, W. E., 1916. Can selection 

 cause genetic change? Amer. Nat., 60: 248-256. Castle, W. E., 1916 a. Fur- 

 ther studies of piebald rats and selection, with observations on gametic coupling. 



