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Proceedings of the Royal Society of Edinburgh. [Sess. 
rendered by Owen to the study of zoology by the introduction of this 
apparently small piece of verbal mechanism ; it takes place with the 
classificatory terms of Linnaeus. And, though the conceptions of ‘ archi- 
typal morphology,’ to which it had reference, are now abandoned in 
favour of a genetic morphology, yet we should remember, in estimating 
the value of this and of other speculations which have given place to 
new views in the history of science, the words of the great reformer 
himself : ‘ Erroneous observations are in the highest degree injurious to 
the progress of science, since they often persist for a long time. But 
erroneous theories, when they are supported by facts, do little harm, 
since everyone takes a healthy pleasure in proving their falsity * 
(Darwin).” 
I have given the above quotation at length partly because of the con- 
cluding sub-quotation. Since reading Lucas’s paper some years ago I 
have more than once endeavoured without success to come to a satis- 
factory decision as to the essential distinction between homology and 
analogy; the terminology presents such difficulties in the case of the 
“ serial homologies ” in the limbs. That there is no absolute distinction 
between the term “ analogy ” and the term “ homology ” (as frequently used) 
has finally been proved to me by a paper of Lankester (1870). 
Lankester finds it necessary to distinguish between different kinds of 
homology, and his distinction is vital. I shall endeavour to condense 
his argument. He first points out that organs may be called homo- 
genetic if the common possessors are derived from ancestors that possessed 
the same organ. But “ homology ” is often used in a wider sense. Thus, 
the four cavities of the bird’s heart had been said to be homologous with 
the four cavities of the mammalian heart, in spite of the fact that the 
common ancestors of mammals and birds had in all probability but three 
heart cavities, and in spite of the further fact that the right ventricle of 
a bird’s heart does not develop in the same way as the right ventricle of 
a mammalian heart. Again, certain muscles in the limbs of Sauropsida 
are still said to be homologous with other muscles in the limbs of 
Mammalia, although the presumption is that no such muscles were 
present in the limbs of the common amphibian ancestors. The crown- 
ing instance, however, is that of the “ serial homologies,” in which a corre- 
spondence is traced in detail between the structures composing, say, the 
fore-limb and those composing the hind-limb of one of the higher verte- 
brates. His conclusion is that something over and above simple Icomogeny 
is in such cases connoted by the term “ homology,” and he proceeds to state 
the question thus : — 
