Although Huxley found no structural differences between the brains of man and of anthropoid apes, he was careful to lay great stress on the important difference in size and weight.  A full-grown gorilla is nearly twice as heavy as a European woman, and yet the heaviest known gorilla brain probably does not exceed twenty ounces in weight, while healthy adult human brains probably never weigh less than thirty-one or thirty-two ounces.  This difference is not of systematic importance; for cranial capacities shew that relatively and absolutely there is a greater difference in brain-weight between the lowest and highest human beings than there is between the highest ape and the lowest human being.

In dealing with the suggestion that man differs from the apes in being bimanous, while the apes are quadrumanous, Huxley first explained and discussed what the exact differences between hands and feet are.  He shewed that in man the foot is absolutely distinguished from the hand by three structural points, although the two organs are similar in general ground-plan.  These structural points are: 

1.  The arrangement of the tarsal bones.

2.  The possession of a short flexor and short extensor muscle of the digits.

3.  The possession of a muscle named peronaeus longus.

Then he described the foot of the gorilla, and shewed that although it was superficially hand-like, it possessed all the structural characters that distinguish a foot from a hand.  Tracing the structure of the foot downwards through the series of anthropoids and monkeys, he established clearly that, while important differences existed in nearly every single creature, the differences between the gorilla and man were not greater than those between the gorilla and other anthropoids, and less than between the gorilla and lower monkeys.

This wonderful series of lectures ranks very high among the important works of Huxley.  It is true that a considerable proportion of the work was not absolutely original, but it had all been specially verified by him.  It was a task undertaken with the greatest courage, and with a care equal to the courage; and it settled conclusively for all time the impossibility of making between man and the anthropoids any anatomical barriers greater than those which exist between the different although closely related members of any of the other family groups in the animal kingdom.  The advance of knowledge has only added to the details of Huxley’s argument; it has not made any reconstruction of it necessary.  A writer on the same subject to-day would to all certainty make use of the same general methods.  The chief differences, perhaps, that would be made are two:  First, greater stress would be laid on the distinction, first made by Huxley himself, between intermediate and linear types. (See p. 87).  To use the popular phrase, a great deal of water has passed under the bridges since the separation of man from the ape-like progenitors common to him