CHAPTER IX 

 VARIATION AND MUTATION 



It becomes imperative that we should carry out the most exact 

 research possible by means of experiment and also wean ourselves of 

 the convenient, but, as it seems to me, highly pernicious habit of theo- 

 retical explanations from general propositions. Otherwise there is 

 great danger that the bright ex])ectation which Darwin has opened out 

 to us by his theory may be baffled — the prospect of gradually ])riugiiig 

 even organic Being within reach of that method of inquiry which 

 seeks to discern mechanical efficient causes. — Semper. 



Thus far in our discussion of evolution factors and theories 

 we have taken for granted the actuality of the two fundamental 

 factors, variation and heredity. No one disputes their reality; 

 nor does an3'one deny their fundr.mental and indispensable 

 character in relation to the origin of species and the evolution 

 of organisms. All the theories to explain evolution build on 

 these two basic factors or vital conditions. The subjects of 

 doubt or denial are such postulated factors as selection, muta- 

 tion, orthogenetic progress, etc.; variation and heredity never. 



But the character, the influence, the regularity or irregu- 

 larity of variations, their behavior in heredity, whether trans- 

 missible or not, whether acquired or congenital, whether deter- 

 minate or indeterminate, etc. — these are the j^roblems that the 

 factor variation or variability presents to biologists. Heredity, 

 too, has its problems. These we shall take u]> in another 

 chapter. 



That variations exist is too ol:)vious to everyone to need any 

 discussion. Any litter of kittens or jiuppies, of mice or pigs, 

 shows us the differences in pattern, shajie, and physiology of in- 

 dividuals born at one time and of the same parents. In wild 

 nature the variations among l^rothers and sisters are no less real 

 than among these domesticated animals. 



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