CHAPTER X. 



WHAT IS THE ORIGIN OF SPECIES?— THE PROBLEM AND 

 ITS EXPLANATION. 



We have seen that there are organic individuals; that they 

 all, however complex their organization, begin as simple cells, 

 and pass through, in each case, definite stages of development, 

 assuming by degrees greater and greater differentiation of the 

 cell. The chief stages of this development are the cellular 

 segmentation, the formation of tissues in embryonic growth, 

 and the attainment of maturity by steps of modification which 

 are in almost every observable particular the exact repetition 

 of steps of modification which their immediate parents passed 

 through in attaining their maturity. 



Variation and Mutability Essential Presumptions in the Discus- 

 sion of Origin of Species. — The differences which the individual 

 presents, when closely compared with its parents, are called 

 variations, or varietal characters. The characters which each 

 individual possesses in common with its parents are classified 

 according to their importance and permanence, and arranged 

 in order from lesser to greater, as specific, generic, family, 

 ordinal, class, or branch characters. 



It is a generally accepted belief that the assumption by the 

 individual of all of the characters which it bears in common 

 with its immediate ancestors is sufficiently accounted for by 

 what are called the natural laws of reproduction; that the 

 slight departure from exact repetition is an insignificant and 

 indeterminate accident of all organisms, or that it is an expres- 

 sion of the imperfection with which the process of reproduc- 

 tion acts. 



The theory of zoologists of the first half of the century 

 was that the species were immutable; that variations were 

 not cumulative, but were always simply variations, the spc- 



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