296 Heredity. 



sible to trace out in its completeness the effect of an ex- 

 ternal influence. A change outside the body may have 

 an obvious and direct effect upon the cells of a certain 

 part, and these cells may influence other cells and so on 

 indefinitely. Any of the cells which are thus affected 

 may give rise to gem mules, and may thus result in a 

 favorable variation which will be seized upon and per- 

 petuated by natural selection. A new variation may 

 therefore follow from an external change which has no 

 direct influence upon the part in which the variation oc- 

 curs. This would be an apparent but not a real objec- 

 tion to our view that the cause of a variation is to be 

 sought in the unfavorable action of changed conditions 

 upon the part in which the variation occurs, but our in- 

 ability to trace the connection between a variation and 

 the external change to which it is due, is no reason for 

 doubting the reality of the connection. 



Saltatory Evolution. 



The origin of species by the natural selection of mi- 

 nute fortuitous variations, demands time which is so 

 long that it is practically infinite, and many naturalists 

 have accordingly held that the successive changes may 

 possibly not be so minute as Darwin believes. Thus 

 Huxley says: " We greatly suspect that Nature does make 

 considerable jumps in the way of variation now and 

 then, and that these saltations give rise to some of the 

 gaps which appear to exist in the series of known forms." 



Galton compares the evolution of an organism to the 

 rolling of a rough stone, which has, in consequence of 

 its roughness, a vast number of natural facets on any 

 one of which it might rest in stable equilibrium. When 

 pushed, this stone would yield a little; but it would fall 

 back again on the withdrawal of the pressure, unless this 



