488 TEXTBOOK OF BOTANY 



today are the modified descendants of preexisting ones, and so on back 

 through milhons of years. Darwin, of course, tried to explain how all 

 these changes in living organisms may have occurred. We may not agree 

 with all his explanations today, for at that time ( middle of 19th century ) 

 nothing was known about chromosomes or their relations to hereditary 

 factors, and there was no clear distinction between heritable and non- 

 heritable variations. 



Mendel did not know what to believe about the various ideas con- 

 cerning hybrids that were current in his time. Two years before Darwin 

 published his book, the Origin of Species, Mendel began his famous 

 experiments with peas. We are already familiar with the definite ratios 

 he discovered through his experiments. Even more important, however, 

 is his inference that hereditary factors must be discrete units that may be 

 combined in cells and later separated without loss of their individuality. 

 At the turn of the century, Bateson of England and de Vries of Holland 

 came to the conclusion, on the basis of experiments, that all hereditary 

 variations occur as if they were the result of abrupt (discontinuous) 

 changes in the cells. The clear recognition of chromosomes in 1884, and 

 the subsequent discoverv of their regular behavior, of their relation to 

 the transmission of hereditary factors, and of their occasional aberra- 

 tions furnished a background of fact adequate to confirm the conclu- 

 sion of Bateson and de Vries. 



This conclusion is quite distinct from Darwin's earlier idea that 

 changes in organisms come about bv the accumulation of slight continu- 

 ous changes occurring in the same direction, and surviving because they 

 are of some "advantage," or of "adaptive value." We have already shown 

 that mutations may be beneficial, harmful, or inconsequential, and that 

 their survival does not depend upon their being beneficial to the plant. 



In the quest for knowledge and understanding of living organisms, if 

 we cannot make pertinent observations and perform critical experiments 

 ourselves, we can at least base our thinking upon the well-attested data 

 discovered by others. 



REFERENCES 



Aase, Hannah C. Cytology of cereals. Bot. Rev. 1:467-496. 1935. 



Blakeslee, A. F. The present and potential service of chemistry to plant breed- 

 ing. Amer. Jour. Bot. 26:163-172. 1939. 



Blakeslee, A. F., and A. G. Avery. Methods of inducing doubling of chromo- 

 somes in plants. Jour. Heredity. 28:393-411. 1937. 



