Thomas Henry Huxley (1825-1895) was an English biologist, often called "Darwin's Bulldog" because of his spirited advocacy of Darwin's theory of evolution. This 1863 book proposed the extension of evolutionary theory to humans, years before Darwin's The Works of Charles Darwin: The Descent of Man, and Selection in Relation to Sex, Volume 22 Part 2 was written in 1871. The book began as an extension of public lectures that Huxley gave.
His argumentation is by analogy: "There is not much apparent resemblance between a barn-door Fowl and the Dog who protects the farm-yard. Nevertheless the student of development finds, not only that the chick commences its existence as an egg, primarily identical in all essential respects with that of the Dog, but that the yelk of this egg undergoes division---that the primitive groove arises, and that the contiguous parts of the germ are fashioned, by precisely similar methods, into a young chick, which at one stage of its existence, is so like the nascent Dog, that ordinary inspection would hardly distinguish the two."
He admits the gap between man and modern apes (the African hominids had not been discovered when this book was written, of course). Nevertheless, "Without question, the mode of origin and the early stages of the development of man are identical with those of the animals immediately below him in the scale---without a doubt, in these respects, he is far nearer the Apes, than the Apes are to the Dog," and "in whatever proportion of its limbs the Gorilla differs from Man, the other Apes depart still more widely from the Gorilla," and as "greatly as the dentition of the highest Ape differs from that of Man, it differs far more widely from that of the lower and lowest Apes."
In a sense, this is his major argument, and it is repeated throughout the book: "there is no existing link between Man and the Gorilla, but do not forget that there is a no less sharp line of demarcation, a no less complete absence of any transitional form, between the Gorilla and the Orang, or the Orang and the Gibbon."
This brief but important work is essential reading for anyone interested in the development of evolutionary theory.