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dragon-flies, which have the seminal vessels in the ninth, while 
the complex male sexual organs are situated in the second, abdo- 
minal segment. It is interesting to note that this curious anomaly 
occurs in an order which is considered to be of the greatest 
antiquity and most generalized type among the true insects. 
There are many other facts of a similar character to those I 
have now touched upon, and they all become clearly intelligible 
on the theory of Mr. Spencer, that the Annulosa are really 
compound animals, or, as he expresses it, “ aggregates of the third 
order ;” while the other great groups of highly organized animals 
—Mollusea and Vertebrata—are typically simple animals, or 
“agoregates of the second order,” (the cells of which their struc- 
tures are built up being “ aggregates of the first order”). Nothing 
of a similar character is to be found among the two latter groups. 
No molluscous or vertebrate animal can be divided transversely 
so that the separate segments shall be in any degree alike, and 
contain repetitions of any important organs. The distinct 
separation of parts in the vertebral column has been acquired, for 
it is less visible in the lower types than in the higher (the reverse 
of what obtains among insects), and in the lowest of all is quite 
absent; while in none is there any corresponding multiplicity or 
displacement of respiratory, circulatory, or generative organs. The 
vertebral column corresponds rather to the segmented shell of the 
Chiton, and has no more relation than it to the essential plan of 
the more important vital organs. Neither does any mollusk or 
vertebrate undergo spontaneous fission, nor that complete and 
progressive segmentation in the process of developmeut which is 
characteristic of all Annulosa; nor do they ever exhibit the pheno- 
mena of parthenogenesis or alternation of generations, the essential 
feature of both which is, that numerous individuals are produced 
from a single fertilized ovum, by a process analogous to (or 
perhaps identical with) ordinary gemmation, and both which pheno- 
mena sometimes occur even among the higher insects. 
In concluding this short sketch of a remarkable theory, I would 
observe, that if it is a true one it at once inyests the objects of our 
study with a new and exceptional interest; because they are the 
most highly developed portion of a group of animals which will, 
in that case, differ fundamentally in their plan of structure from 
all other highly organized forms of life. In the study of the 
habits, instincts, and whole economy of insects, we shall have to 
