54 
Herbert Spencer's account of the causes which, as he con- 
ceives, produce differentiation in a homogeneous aggregate. 
He enumerates, in his chapter on the instability of the homo- 
geneous, several examples of it both from mechanics and from 
chemistry. It is not necessary to follow him through those 
examples ; but if I do not mistake, they all seem to me to be 
chargeable with one notable defect, namely, that they all pre- 
suppose a differentiation of some kind, and therefore are nob 
cases of a departure from a primitive homogeneous state at 
all. For instance, in the case of water in a state of complete 
quiescence, and of equal density throughout (supposing this 
possible), he says: ff The radiation of heat from neighbouring 
bodies, by affecting differently its different parts, would inevi- 
tably produce inequalities of density and consequent currents ; 
and would so render it to that extent heterogeneous." But 
surely the radiation of heat from neighbouring bodies pre- 
supposes, first, that there are bodies separate, and therefore 
differentiated, from the water; and secondly, that these bodies 
are hotter than the water — another differentiation . And again, 
he instances the oxidation of metal when exposed to air or 
water as an example of the change from homogeneity to 
heterogeneity. But this again pre-supposes a difference 
already existing between the metal and the air or water which 
acts upon it. The same defect seems to run through all his 
examples : but inasmuch as he afterwards gives a general 
explanation as applicable to every case, we need no longer 
delay upon the particular examples, but proceed to consider 
that general explanation. His words are these : — The in- 
stability thus variously illustrated is obviously consequent on 
the fact that the several parts of any homogeneous aggrega- 
tion are necessarily exposed to different forces — forces that 
differ either in kind or amount ; and being exposed to different 
forces they are of necessity differently modified. The relations 
of outside and inside, and of comparative nearness to neigh- 
bouring sources of influence, imply the reception of influences 
that are unlike in quantity or quality, or both ; and it follows 
that unlike changes will be produced in the parts thus dis- 
similarly acted upon." 
16. Here Mr. Spencer divides these supposed forces into two 
classes: those that differ in kind, and those that differ in 
amount. It is at once evident that in a homogeneous whole 
there could be no forces differing in kind, for the simple 
reason that if there were the aggregate would not be homo- 
geneous. For the same reason there could be no forces dif- 
fering in amount, except from differences of distance. Bishop 
Cotterill says, in the paper lately referred to, that if we sup- 
