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and its Significance in Systematic Zoology. 89 
and a Quadruped, and yet we perceive between them close re- 
semblances of form ; but between a Worm and a Siphonops, or 
between an Insect and a Bird, we can readily argue a community, 
because we at once estimate the narrow limits in the one case, 
and the wide extent in the other, of their analogical functions. 
It would be highly unphilosophical to suppose that these close 
resemblances were the effect of accident, and still more so to say 
that they result from accident in one case, and from profound 
design in another. 
The homomorphisms existing between the Vertebrata and 
Invertebrata are not numerous; indeed, as might be expected in 
animals so widely separated, they are rare, and usually im- 
perfect. I confess they present the greatest difficulty ; and yet, 
where knowledge of habit assists us, the difficulty to a great 
extent vanishes. There is no Class of Invertebrata more fami- 
liarly known than the insects, and there are no clearer homo- 
morphisms between these great subkingdoms than those between 
insects and birds ; and who is there that does not perceive that 
the forms assumed by insects are as much the necessity of their 
habits, and that in habits, as in form, they assimilate to birds, 
just as a Bat does, or as a Whale agrees with a fish. 
Again, how little do we know of the habits of the Invertebrate 
classes generally ? The majority of them are marine ; and it is 
only quite recently that they have even been seen, except through 
the medium of pictures, by the majority of persons. We are not 
on terms of familiarity with chem, as we are with quadrupeds 
and birds; and seeing that our comprehension of their homo- 
morphism i is in direct ratio to our knowledge of their habits and 
modes of life, it is not a matter of surprise that we should be 
unable to penetrate the mystery of the similarity between the 
Foraminifera and the Mollusca, or between the Polypes and the 
Polypine Infusories. For here again the explanation of their 
homomorphism is measured by the amount of our knowledge. 
We see why a Bombylius resembles a Bombus, or a Ter edo a 
Sabella, having some acquaintance with the similar habits of 
each, and seeing a degree of similarity between them. We know 
why a Caddis-worm resembles a Tubicolous Annelide, and this, 
again, a tube-inhabiting Rotifer ; it is the common habit of form- 
ing a tube for their otherwise unprotected body which assimilates 
them; but we know not why a Chiton resembles an Aphrodite, 
because we are equally ignorant of the habits of either. 
Let me now, in application of the foregoing principles, throw 
out some suggestions in relation to the most striking instance of 
homomorphism which occurs, perhaps, in the animal kingdom 
—viz. that existing between the Polyzoan Molluscoids and the 
Hydroid Polypes.. In both these widely-separated groups, we 
