CHAPTER XL 



THE THEORY OF HEREDITY CONSIDERED AS SUPPLEMEif- 

 TARY TO THE THEORY OF NATURAL SELECTION. 



Darwin believes that variations are purely fortuitous — Natural 

 selection cannot give rise to permanent race modifications un- 

 less many individuals vary in nearly the same way, at about 

 the same lime — The chances against this are very great if 

 variations are fortuitous — Argument from North British Re- 

 view — Darwin acknowledges the great weight of this objection 

 — It is removed by the theory of heredity — The co-ordinated 

 modification of complicated organs — Tlie time demanded by 

 Darwin practically infinite — Murphy's argument from the 

 complexity of the eye — Herbert Spencer's illustration — Our 

 theory removes this difficulty — Mr. Conn's objection — Salta- 

 tory evolution — Evidence that it occurs — Spike horn buck — 

 Ancon and Mauchamp sheep— Black-shouldered peacock — 

 The theory of heredity accounts for saltatory evolution — 

 Parallel variation — Evidence of its occurrence — Evolution of 

 the medusae — General and special Homologies. 



According to Darwin's view, yariations, though deter- 

 mined by definite causes (for the most part unknown), 

 are, so far as their usefulness to the organism goes, for- 

 tuitons, and he makes use of the following illustration 

 to explain his conception: 



**I have spoken of selection as the paramount power, 

 yet its action absolutely depends upon what we in our 

 ignorance call spontaneous or accidental variability. 

 Let an architect be compelled to build an edifice with 

 uncut stones, fallen from a precipice. The shape of 

 each fragment may be called accidental, yet the shape 

 of each has been determined by the force of gravity, the 



