36 EVOLUTION AND THE 
forms of it which we can know or conceive—must in- 
evitably lapse into heterogeneity. In three several 
ways does the persistence of force necessitate this, 
Setting external agencies aside, each unit of a homo- 
geneous whole must be differently affected from any 
of the rest by the aggregate action of the rest 
upon it. The resultant forces exercised by ‘the 
aggregate on each unit, being in no case alike 
both in amount and direction, cannot produce like 
effects on the units. And the various positions 
of the parts in relation to any incident force pre- 
venting them from receiving it in uniform amounts 
and directions, a further difference in the effect . 
wrought on them is inevitably produced.” Even 
this is not all: Mr. Spencer also points out that “every 
differentiated part is not simply a seat of further dif- 
ferentiations, but also a parent of further differentia- 
tions; since in growing unlike other parts, it becomes 
a centre of unlike reactions on incident forces, and by 
so adding to the diversity of forces at work adds to the 
diversity of effects produced. This multiplication of 
effects is proved to be similarly traceable throughout 
Nature.” Now, if causes like these are inevitably at 
work upon and within the simplest forms of life, no 
change in external conditions would be needed in order 
to insure alteration or increasing complexity of struc- 
ture, through months or years, to say nothing about 
