The Making of Species 



By inheritance we mean that which an organ- 

 ism receives from its parents and other ancestors 

 all the characteristics, whether apparent or 

 dormant, it inherits or receives from its parents. 

 Professor Thomson's definition "all the qualities 

 or characters which have their initial seat, their 

 physical basis, in the fertilised egg cell " seems 

 to cover all cases except those where eggs are 

 parthenogenetically developed. 



The first fact of heredity which we must notice 

 is that inheritance may take several forms. This 

 is apparent from what was set forth in the 

 chapter dealing with hybrids. 



In considering the phenomena of inheritance 

 it is convenient to deal with crosses in which the 

 parents do not closely resemble one another, 

 because by so doing we are able readily to 

 follow the various characters displayed by each 

 parent. It may, perhaps, be urged that such 

 crosses occur but rarely in nature. This is true. 

 But we should bear in mind that any theory 

 of inheritance must explain the various facts of 

 cross-breeding, so that, from the point of view of 

 a theory of inheritance, crosses are as important 

 as what we may term normal offspring. As 

 inheritance is so much easier to observe in the 

 former, it is but natural that we should begin 

 with them. Our deductions must, if they be 

 valid ones, fit all cases of ordinary inheritance, 

 i.e. all cases where the offspring results from the 



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