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is to me surprising that one of the most ingenious and remarkable 
theories ever put forth on a question of Natural History has not 
been so much as once alluded to. More than six years ago, 
Mr. Herbert Spencer published, in his ‘ Principles of Biology,’ a 
view of the nature and origin of the Annulose type of animals, which 
goes to the very root of the whole question; and, if this viewisasound 
one, it must so materially affect the interpretation of allembryologi- 
caland anatomical facts bearing on this great subject, that those who 
work in ignorance of it can hardly hope to arrive at true results. 
I propose, therefore, to lay before you a brief sketch of 
My. Spencer’s theory, with the hope of calling attention to it, and 
inducing some of you to take up what seems to me to be a most 
promising line of research; and, although the question is one on 
which I feel quite incompetent to form a sound judgment, I shall 
call your attention to the light which it seems to throw on some 
of the most curious anomalies of insect structure. 
The theory itself may be enunciated in very few words. It is, 
that insects, as well as all the Annulosa, are not primarily 
single individuals, but that each one is a compound, representing 
as many individuals as there are true segments in the body, these 
individuals having become severally differentiated and specialized 
to perform certain definite functions for the good of the whole 
compound animal. 
Mr. Spencer first calls attention to the fact, that among the 
undoubtedly compound animals (which are almost all found in 
the sub-kingdoms, Coelenterata and Molluscoida) the several in- 
dividuals are rarely combined in such a manner as to necessitate 
any physiological division of labour among them. The associated 
individuals of a Hydrozoon or an Ascidian are each free to spread 
their tentacles, to draw in currents of water, and to select 
their food, without in any way interfering with each other, 
because the compound animal is either branched or approximately 
hemispherical, and thus there is no necessity for any of the 
combined individuals to become especially modified with regard 
to the rest. But should a compound animal have its component 
individuals arranged in a linear series, there would most probably 
arise a marked difference of conditions between the two situated 
at the extremities and those between them. If they remained 
united, some modification must have occurred to adapt each to its 
condition. But if, further, the series should be fixed at one end, 
