The necessity which is felt of limiting the comparison of cerebral and psychical development to individuals of the same species* proves that some other condition than size is essential in determining the energy of each faculty, and that peculiarity of form and position of parts of the brain may give the external appearance of excess or deficiency of size ; admissions which must throw doubt on the sufficiency of the only means which phrenology possesses of collecting facts to support its theory. If structure and form render it impossible by the size alone to determine the comparative psychical characters of two individuals of different species, the same conditions may often disturb the results that should proceed from comparisons of size in the organs of two of the same species.
In the limited field of the comparison of different individuals of the same species, phrenology is said by its supporters to be established by numberless facts of the correspondence of strength or weakness of each of the faculties with proportionate extent or deficiency of develop ment of its organ ; and they maintain that conclusions thus drawn from facts can be overthrown only by facts that contradict them.* But the numerous sources of fallacy which render the presumed facts for phrenology doubtful, present as great an obstacle to the collection of facts against it; and although its theory, which possesses so much plausibility, cannot be altogether overthrown by anything but well ascertained facts, yet it is fair for any one to withhold his assent to it till he believes that it is supported by a sufficient number of positive and unerring observations ; and the more so when he finds it opposed by some, however few, facts, and incapable of explaining several circumstances that might be expected to be placed under its laws. For these reasons the fallacies to which craniological observation is subject must be admitted as casting doubt upon the testimony of phrenologists, who, granting that they are unwilling to deceive, may yet, like all other observers, be charged with the liability to be them selves deceived. We believe that in this we do not step at all beyond the commonly adopted rules of judging from evidence, according to which it is open to every one to disbelieve statements and conclusions that seem to him improbable, although he may not be in a position to disprove them all by facts. .
Judging by these rules, the very perfection to which phrenology is supposed*to have nearly attained is strong evidence of its improbability. No one will deny that, in its connection with the body, the human mind must be the most difficult branch of physiology. Yet if phre nology be true, the physiology of the brain is more advanced than that of any other organ in the body ; and the improbability is presented that two physiologists accomplished more in about 50 years of study of the most difficult branch of the science of life than the united generations of physiologists of all classes for near 2000 years have effected in the most easy. There is not one function of the living body
which can be so perfectly illustrated by the most accomplished physiologists as (if phrenology be true) the functions of the mind can be elucidated by a mediocre adherent to its doctrines; and this too while, to every source of fallacy which it has in common with other departments of physiology, it is subject to still greater which are peculiar to itself.
The improbability that the labour bestowed upon phrenology should have had so extraordinary a result, will appear greater when some of the fallacies are pointed out to which the observations that serve for its basis are subject. Admitting that the size of a part of the brain may be taken as a measure of the power of the faculty of which it is presumed to be the organ, it can be a correct measure of power only when all other conditions are the same. This is admitted by phreno logists, who maintain only that, ceteris paribus, size is a measure of power ; and it may be admitted that in this proposition they are supported by the analogy of other organs of the body. But in all of them, and therefore probably in the brain, the other conditions are fully as important as size ; yet phrenologists in their usual practice refer to quality of the brain only when they find that the indications of quantity are manifestly opposed to their opinions. Moreover the estimate which phrenologists make of the quality of the brain, by observing the external appearance and temperament of the individual, is as fallacious as a measure of the state of the whole mass, and is valueless as a sign of the structural condition of each of its several parts. But any one part of the brain may as well differ from the rest in quality as in quantity; an assumption which the phenomena of local diseases, which are much more common than general diseases of the brain, are sufficient to establish, and which phrenologists them selves admit in their explanation of monomania. There is here therefore a manifest source of fallacy in every phrenological observa tion ; a source of fallacy comparable with, but greater than, most of those which have so long obscured the knowledge of the more simple departments of physiology. No one who has had any opportunity of appreciating the difficulty of analysing observations of which such a varying source of error as is here indicated forms a constant part, can avoid suspecting that phrenologists, when they pretend to have overcome the difficulty, have merely wandered into the facilities of error.