The Making of Species 



think it more probable that all species throw off 

 at greater or less intervals discontinuous varia- 

 tions, and that it is upon these that natural selec- 

 tion acts. 



We further hope that we have succeeded in 

 making clear what we believe to be the very 

 sharp distinction between continuous and dis- 

 continuous variations, even when the latter are 

 inconsiderable, as frequently happens. 



Before leaving the subject of variation it is 

 necessary to notice the distinction, which Weis- 

 mann was the first to emphasise, between somatic 

 and germinal variations. 



Every adult organism must be regarded as the 

 result of two sets of forces ; inherited tendencies 

 or internal forces, and the action of environment 

 or external forces. The differences which the 

 various members of a family show are due in 

 part to the initial differences in the germinal 

 material of which they are composed, and in part 

 to the differences of their environment. The 

 former differences are the result of what we may 

 call germinal variations, and the latter the result 

 of somatic variations. It is scarcely ever possible 

 to say of any particular variation that it is a 

 germinal or a somatic one, because even before 

 birth a developing organism has been subjected 

 to environmental influences. One of a litter may 

 have received more nourishment than the others. 

 Nevertheless, any marked variation which appears 



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