158 The Bible of Nature 



a heavy burden on the shoulders of natural se- 

 lection. 



The Organism is a Unity. (3) It is also becoming 

 more and more evident that the living creature 

 varies, in many cases, as a unity. If there is more 

 of one character, there may be less of another; one 

 change brings another in its train. As Darwin 

 pointed out, there is a "correlation of variation." 

 We see one part varying and we can plausibly say 

 that its changes in a given direction are useful and 

 life-preserving, but meanwhile there may be in the 

 train of this observable variation another which is 

 destined to be of far greater import. Another 

 aspect of the same idea, illustrated for instance 

 by the authors of "The Evolution of Sex," 1 is that 

 changes apparently confined to minute and super- 

 ficial parts may be, as it were, the correlated out- 

 crop of deeper physiological variations of the whole 

 system or of a large part of the system. As Pro- 

 fessor Ray Lankester says, 2 ' ' We should, perhaps, 

 more generally conceive of variation as not so much 

 the accomplishment and presentation of one little 

 mark or difference in weight, length, or color, as the 

 expression of a tendency to vary in a given tissue or 

 organ in a particular way. Thus we are prepared 



1 P. Geddes and J. Arthur Thomson, " The Evolution of 

 Sex," " Contemporary Science Series," Revised Edition, 

 1901. 



2 "The Kingdom of Man," 1907, p. 132. 



