It appears that Lamarck's doctrine in the Philosophie Zoologique (1809)[(4)] made a profound impression on the mind of Lyell, who was the first to treat the descent of man in a broad way from the standpoint of comparative anatomy and of geologic age. In his great work of 1863, The Geological Evidences of the Antiquity of Man, Lyell cited Huxley's estimate of the Neanderthal skull as more primitive than that of the Australian but of surprisingly large cranial capacity. He concludes with the notable statement: "The direct bearing of the ape-like character of the Neanderthal skull on Lamarck's doctrine of progressive development and transmutation ... consists in this, that the newly observed deviation from a normal standard of human structure is not in a casual or random direction, but just what might have been anticipated if the laws of variation were such as the transmutationists require. For if we conceive the cranium to be very ancient, it exemplifies a less advanced stage of progressive development and improvement."[(5)]
Lyell followed this by an exhaustive review of all the then existing evidence in favor of the great geological age of man, considering the 'river-drift,' the 'loess,' and the loam deposits, and the relations of man to the divisions of the Glacial Epoch. Referring to what is now known as the Lower Palæolithic of St. Acheul and the Upper Palæolithic of Aurignac, he says that they were doubtless separated by a vast interval of time, when we consider that the flint implements of St. Acheul belong either to the Post-Pliocene or early Pleistocene time, or the 'older drift.'
It is singular that in the Descent of Man, published in 1871,[(6)] eight years after the appearance of Lyell's great work, Charles Darwin made only passing mention of the Neanderthal race, as follows: "Nevertheless, it must be admitted that some skulls of very high antiquity, such as the famous one at Neanderthal, are well-developed and capacious." It was the relatively large brain capacity which turned Darwin's attention away from a type which has furnished most powerful support to his theory of human descent. In the two hundred pages which Darwin devotes to the descent of man, he treats especially the evidences presented in comparative anatomy and comparative psychology, as well as the evidence afforded by the comparison of the lower and higher races of man. As regards the "birthplace and antiquity of man,"[(7)] he observes:
"... In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject; for two or three anthropomorphous apes, one the Dryopithecus of Lartet, nearly as large as a man, and closely allied to Hylobates, existed in Europe during the Miocene Age; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.
"At the period and place, whenever and wherever it was, when man first lost his hairy covering, he probably inhabited a hot country; a circumstance favorable for the frugivorous diet on which, judging from analogy, he subsisted. We are far from knowing how long ago it was when man first diverged from the catarrhine stock; but it may have occurred at an epoch as remote as the Eocene Period; for that the higher apes had diverged from the lower apes as early as the Upper Miocene Period is shown by the existence of the Dryopithecus."
With this speculation of Darwin the reader should compare the state of our knowledge to-day regarding the descent of man, as presented in the first and last chapters of this volume.
The most telling argument against the Lamarck-Lyell-Darwin theory was the absence of those missing links which, theoretically, should be found connecting Man with the anthropoid apes, for at that time the Neanderthal race was not recognized as such. Between 1848 and 1914 successive discoveries have been made of a series of human fossils belonging to intermediate races: some of these are now recognized as missing links between the existing human species, Homo sapiens, and the anthropoid apes; and others as the earliest known forms of Homo sapiens:
| Year | Locality | Character of Remains | Race |
| 1848 | Gibraltar. | Well-preserved skull. | Neanderthal. |
| 1856 | Neanderthal, near Düsseldorf. | Skullcap, etc. | Type of Neanderthal race. |
| 1866 | La Naulette, Belgium. | Fragment of lower jaw. | Neanderthal race. |
| 1867 | Furfooz, Belgium. | Two skulls. | Type of Furfooz race. |
| 1868 | Crô-Magnon, Dordogne. | Three skeletons and fragments of two others. | Type of Crô-Magnon race. |
| 1887 | Spy, Belgium. | Two crania and skeletons. | Spy type of Neanderthal race. |
| 1891 | Trinil River, Java. | Skullcap and femur. | Type of Pithecanthropus race. |
| 1899 | Krapina, Austria-Hungary. | Fragments of at least ten individuals. | Krapina type of Neanderthal race. |
| 1901 | Grimaldi grotto, Mentone. | Two skeletons. | Type of Grimaldi race. |
| 1907 | Heidelberg. | Lower jaw with teeth. | Type of Homo heidelbergensis. |
| 1908 | La Chapelle, Corrèze. | Skeleton. | Mousterian type of Neanderthal race. |
| 1908 | Le Moustier, Dordogne. | Almost complete skeleton, greater part of which was in bad state of preservation. | Neanderthal. |
| 1909 | La Ferrassie I, Dordogne. | Fragments of skeleton. | Neanderthal. |
| 1910 | La Ferrassie II, Dordogne. | Fragments of skeleton, female. | Neanderthal. |
| 1911 | La Quina II, Charente. | Fragments of skeleton, supposed female. | Neanderthal. |
| 1911 | Piltdown, Sussex. | Portions of skull and jaw. | Type of Eoanthropus, the 'dawn man.' |
| 1914 | Obercassel, near Bonn, Germany. | Two skeletons, male and female. | Crô-Magnon. |
In his classic lecture of 1844, On the Form of the Head in Different Peoples, Anders Retzius laid the foundation of the modern study of the skull.[(8)] Referring to his original publication, he says: "In the system of classification which I devised, I have distinguished just two forms, namely, the short (round or four-cornered) which I named brachycephalic, and the long, oval, or dolichocephalic. In the former there is little or no difference between the length and breadth of the skull; in the latter there is a notable difference." The expression of this primary distinction between races is called the cephalic index, and it is determined as follows:
Breadth of skull × 100 ÷ length of skull.