THE PllINCIPLES OF BIOLOGY. 
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to establish types in which the connected individuals are 
more unlike one another, at the same time that their several 
individualities are most distinguished by the integration 
consequent on their mutual dependence.” In the Annulosa 
and especially in the Annuloula , from “ traits of structure 
development and mode of multiplication,” it is shown that 
“ every segment is in great measure a physiological whole— 
every segment contains most of the organs essential to 
individual life and multiplication; such essential organs as 
it does not contain being those which its position as one in 
the midst of a chain prevents it from having or needing.” 
The Annulosa are therefore aggregates of the third order. 
In approaching the consideration of a still higher division 
—the Mollusca —Mr. Herbert Spencer reverts to a previous 
section, and emphasizes the truth that “ As before explained 
under the head of Classification, organisms do not admit 
of uni-serial arrangement either in general or in detail; but 
everywhere form groups within groups. Hence having 
traced the phases of morphological composition up to the 
highest forms in any sub-kingdom, we find ourselves at the 
extremity of a great branch, from which there is no access 
to another great branch, except by going back to some place 
of bifurcation low down in the tree.” The Mollusca differ 
materially from their allies the Molluscoida —both considered 
as groups—in that they are single and not compound. “ No 
true Mollusk multiplies by gemmation, either continuous or 
discontinuous; but the product of every fertilized germ is a 
single individual.” The significant fact is then dwelt upon 
that liomogenesis holds throughout an entire sub-kingdom, 
and that “ there is no case in which the organism is divisible 
into two, three, or more like parts. There is no clustering 
as in the Ccdenterata or segmentation as in the Annulosa, 
the simulation of segmentation by one of the group of the 
Mollusca —the Chiton —being limited to the shell only, and to 
this segmentation being adaptive instead of genetic. The 
conclusion arrived at is therefore that a Mollusk is an 
aggregate of the second order. “Not only in the adult 
animals is there no sign of a multiplicity of like parts that 
have become obscured by integration ; but there is no sign 
of such multiplicity in the embryo.” 
In the highest sub-kingdom—the Vertebrata —as among 
the Mollusca , homogenesis is universal. 'The two highest 
sub-kingdoms “ are like one another and unlike the remaining 
sub-kingdoms in this, that in all the types they severally 
include a single fertilized ovum produces only a single 
individual.” Occasional spontaneous fission of the vitelline 
