Huxley believed, as did many scientists at the time, that the evolution of a fetus mirrored that of the evolution of the species. Therefore, by examining the brains of apes and humans before full term, one could deduce the probability that they shared a common ancestor.

The joy in reading the article is not simply historic scientific interest, but the insight we get into the scientific method. Here is an important scientific discussion in play well before our modern scientists could positively confirm or refute many of the items that Huxley and his contemporaries could only guess at. Listen, for example, to Huxley's politely cutting remark about the French scientist Louis Gratiolet, spoken as only an English gentleman could:

>>Gratiolet opens his preface with the aphorism: "It is dangerous to jump to conclusions too quickly in science". I fear he must have forgotten this sound maxim by the time he had reached the discussion of the differences between men and apes, in the body of his work. No doubt, the excellent author of one of the most remarkable contributions to the just understanding of the mammalian brain which has ever been made, would have been the first to admit the insufficiency of his data had he lived to profit by the advance of inquiry.<<

The full title of "On The Brain" is: "Note on the Resemblances and Differences in the Structure and the Development of the Brain in Man and Apes". It was originally printed as twelve pages at the end of Chapter 7 in the 1871 edition of Charles Darwin's "The Descent of Man and Selection in relation to Sex". They don't write titles like that any more.