lo Outline of Genetics 



the others. While there is little question that natural 

 hybridization takes place and may be a real factor in 

 producing new varieties, at the same time this theory 

 is not satisfactory as a ''complete" explanation of evo- 

 lution. It seems rather obvious that, although hybridi- 

 zation can multiply variations through crossing forms 

 that are already different from each other, it can never 

 account for the "original" differences. 



In considering the relative merits of these different 

 explanations, there are three things that it is useful 

 to bear in mind. First, it is certainly not necessary to 

 subscribe to a belief in any one of the theories to the 

 complete exclusion of the others. It is quite possible 

 that every one of them may be a factor in evolution, 

 and altogether probable that no one of them by itself 

 can adequately account for all of the evolutionary change 

 that has taken place. 



Second, it is not advisable to contrast these explana- 

 tions as though they were coordinate units. The "prob- 

 lem of evolution" is not a single problem, but a complex 

 of numerous ones, and any proposed explanation of 

 evolution is confronted by the necessity of answering 

 several distinct questions. The conspicuous questions 

 to be answered are: 



1. What is the cause of variation? 



2. What is the nature of the variations that are 

 important in evolution? 



3 . How may variations be perpetuated and multiplied ? 



4. How are the variations manipulated to effect 

 progressive evolution? 



It will be noticed that Lamarck goes farther than 

 any other author in answering question i. Orthogenesis 



