678 Studies in the Theory of Descent. 



organism the power of giving rise by multipli- 

 cation only to exact copies of itself, or, more 

 correctly, the power of transmitting unaltered to 

 its successors the motion of its own course of 

 development, each " individual variation " must 

 depend upon the power of the organism to react 

 upon external influences, i. e. to respond by 

 changes of form and of function, and conse- 

 quently to modify its original (inherited) develop- 

 mental direction. 



It has sometimes been insisted upon, that the 

 " individuals of the same species " or the offspring 

 of one mother cannot be absolutely equal, because, 

 from the commencement of their existence, they 

 have been subjected to dissimilar actions of the 

 environment. But this implies that by perfectly 

 equal influences they would become equal, i.e. it 

 supposes that variability is not inseparably bound 

 up with the essence of the organism, but is only 

 the consequence of developmental tendencies 

 which are in themselves equal being unequally 

 influenced. As a matter of fact the first germs of 

 an individual certainly cannot be supposed to be 

 perfectly equal, because the individual differences 

 of the ancestors must be contained therein in 

 different degrees according to their constitution, 

 and we should have to go back to the primordial 

 organism of the earth in order to find a perfectly 

 homogeneous root, a tabula rasa from which the 

 descendants would commence their development. 



