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them in its turn.” And hence, all through nature, at once the per- 
manence and intransmutability of species, whether organized or mo- 
lecular, and the construction of these species into genera or groups 
into which the same type enters. 
His first application of this law is to show the genesis of inertia. 
Thus, let any portion or particle of substance considered merely as such 
be subject to this law alone, it follows, that in its relations both to space 
and time, that portion or particle must assimilate itself to itself in 
every successive moment ; that is, if it be at rest it must continue at 
rest, and if it be in motion it must, in every successive moment, ac- 
complish the same element of motion which it accomplished in the 
first ; whence it is obvious that its motion must be uniform and 
rectilinear, and must continue so till it be changed from without. But 
this is simply a definition of inertia, whose genesis thus appears. 
Similarly he shows, that if the portion or particle of substance 
be considered, not in reference to its matter but its form , it must, 
under the law of assimilation, tend to maintain its true volume and 
form, and, to restore it when it has been disturbed — it must be 
resilient and elastic. 
Again, let there be two or more particles or masses (each and 
all of which must thus be inert, and in their last elements elastic also), 
and let them be placed in different regions of space ; it follows, that 
though as to inertia and elasticity they are identical already, and the 
law of assimilation has exhausted itself upon them as substances and 
forms, yet they are not assimilated as to the spaces they occupy. This, 
therefore, the law of assimilation must tend to effect. Under this law 
they must tend towards one place. Moreover, in being assimilated as 
to place, they must also be assimilated as to time. In a word, they 
must tend to move, and that simultaneously, towards one another, each 
assimilating the other as to the place which it occupies, or attracting it 
with a force proportional to itself (its quantity of substance, mass, or in - 
ertia). Nor is it merely the fact of mutual attraction that follows 
from the law of assimilation, but also the law of that attraction. 
Thus, considered as an attractive apparatus, it must be conceived as in- 
vesting in a spherical manner the attracting particle. Now, the law 
of assimilation requires, with respect to such a spherical investi- 
ture, that all the successive spherical shells or superficies, of which 
the entire sphere of attraction may be conceived to consist, must in 
attractive value be assimilated to each other ; that is, they must all be 
