* CHAPTER VII 



ORIGIN OF ADAPTATIONS AND OF SPECIES 



i, ADAPTATIONS 



Organic Evolution. Organic evolution is essentially the 

 evolution of adaptive structures and functions. There remain 

 to be explained, however, non-adaptive structures and func- 

 tions, and no theory of evolution is adequate which does not 

 account for the useless as well as the useful characters. 



Existing structures are due to the nature of the organism 

 and the nature of the environment ; in other words, are results 

 of the activity of protoplasm under the influence of environ- 

 mental forces. Variations arise which are useful or not and 

 either transmissible or not. Useful transmissible variations 

 not only remain but tend to become more nearly perfect ; while 

 useless variations tend to disappear. 



The various theories of organic evolution differ chiefly in 

 their answers to these questions: (i) What is the nature of 

 variations and how do they arise? Variations are .classed as 

 either continuous or discontinuous; adaptive or unadaptive. 

 In asexual organisms, variations are brought about by the 

 direct influence of temperature, light and other primary fac- 

 tors upon protoplasm ; in sexual organisms, variations are due 

 to another cause as well, namely, the union of two kinds of 

 protoplasm. In any given case of variation, how much is due 

 immediately to protoplasm and how much to the environment ? 

 (2) What kinds of variations are transmissible? Discontinu- 

 ous variations (sports) are strongly transmissible as a rule, 

 while continuous (individual) variations are often non-trans- 

 missible ; though it is often difficult to decide whether they are 

 transmissible or not. Each kind of variation has to be exam- 

 ined separately, on its own merits. Difficulties arise from the 



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