PREFACE. 
NEW idea may spring from many other sources besides that of the real or truthful, and may agree 
A with many other bents of the understanding besides that of reason; but a new idea in Science 
must be of a class sai generis, for however completely or imperfectly it may accord with any other 
mental conceit, whether of Poetry or History, it cannot disagree with the natural or exist incompatible 
with the reason, whose basis is fact and reality. It is the character of Science that all its opinions 
‘are freely submitted to demonstration ; its theories do not shun a close examination, or if they do 
not bear of this rigorous test, then we change the name theory to the name hypothesis, and this 
latter is nothing but vacuum, it has not even the materiality of the intrenchant air. The theory 
contrasted with hypothesis manifests as prominent a distinction of species as is apparent between the 
Newtonian system and the Miltonian rhyme: they are like two parallel lines, destined never to meet ; 
and if their species could by any possibility be mingled, the product would prove to be a mere barren 
hybrid, such as the work of the speculative visionary. 
The mind balances between truth and error, between Science and the purely Imaginative ; and by 
experience it proves that every accession to Science causes an abatement in the same ratio to error. 
The more self-evident the truth is, the more apparent the error proves to be; for if all the points in the 
circumference of a circle can be proved to stand equidistant from the centre, then every opinion which 
runs counter to this particular one must be false. Again, if the law of gravitation yields its own 
proof of correctness, it will be in vain that we try to reconcile with the same evidence the idea 
expressed in the line “and to this hour down had been falling.” As facts, therefore, are of a species 
distinct and incompatible with falsities,. it will be to no effect that we try to fuse them by words or 
fancies into one species. The operation of a natural law cannot be controlled by the mere phrase of 
speech. A solution of sulphate of lime, when poured into another solution of carbonate of potash will 
produce a phenomenon of elective affinity which the voice of Stentor will not prevail against, because 
words have no power against facts ; and it is well for Truth and Nature that it is so. The like observation 
applies to the subject of an anatomical unity of organisation, for it is evident that all the worded philosophy 
which has been shed down heretofore upon this theme has not rendered skeleton quantities uniform with 
each other, and the reason is because the fact is otherwise. They are not uniform because they are not 
equals, and therefore they cannot be read as uniform according to reason because they are not equal 
according to Nature. The germ of any new Theory respecting the law of formation which presides over 
skeleton development must take root in this fact of inequality, and grow from it as from a substratum. Before 
we can develope the ideas of unity between two or more forms which are actually unequal as to quantity, 
and only in this respect various to each other, we must first of all know the cause of their inequality. Now 
it requires as much, nay even more, labour to remodel an old-fashioned edifice, in order to make it agree 
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