5 1 8 Studies in the Theory of Descent. 



of the systems of organs and parts of the body 

 becomes difficult and uncertain : still it may 

 safely be asserted that the two Cladocerous 

 families Polyphemidcs and Daphniidce differ much 

 less from one another in the structure of their oar- 

 like appendages than in that of their other parts, 

 such as the head, shell, legs, or abdominal 

 segments. In systematic groups of a still higher 

 order, i.e. in orders, and still more in classes, we 

 might be inclined to consider that all the organs 

 had become modified to an equally great extent. 

 Nevertheless it cannot be conclusively said that 

 the kidneys of a bird differ from those of a 

 mammal to the same extent as do the feathers 

 from mammalian hair, since we cannot estimate 

 the differences between quite heterogeneous things 

 it can only be stated that both differ greatly. 

 Here also the facts are not such as would have 

 bten expected if transformation was the result of 

 an internal developmental force ; no uniform 

 modification of all parts takes place, but first one 

 part varies (variety) and then others (species), and, 

 on the whole, as the systematic divergence in-, 

 creases all parts become more and more affected 

 by the transformation and all tend continually to 

 appear changed to an equal extent. This is pre- 

 cisely what would be expected if the transforming 

 impulses came from the environment. An equal- 

 ization of the differences caused by transformation 

 must be produced in two ways ; first by correlation, 



