Supplement to ''Nature!' August 18, 1923 



267 



" A man in his development runs for a little while 

 parallel with, though never passing through, the form 

 of the meanest worm, then travels for a space beside 

 the fish, then journeys along with the bird and the 

 reptile for his fellow-travellers ; and only at last, 

 after a brief companionship with the highest of the 

 four-footed and four-handed world, rises into the 

 dignity of pure manhood." 



It is true that we cannot explain the infinity of stages 

 passed through by a human embryo, from the fertilised 

 ovum, representing the lowest unicellular stage of living 

 ) things, to the fully formed child, unless we believe that 

 1 man, like all animals, has been evolved from the 

 I simplest of beginnings. But every one of these tran- 

 ' sitional stages represents a new form of being, never 

 one of which has been seen at any stage of the world's 

 history leading an independent adult existence. Every 

 organ and part of the human body passes through 

 an extensive series of developmental changes which 

 receive a full and adequate explanation from the theory 

 of evolution, but not one of these changes, from the 

 first to the last, copies a form seen in any adult animal ; 

 at every point of development old or recapitulatory 

 phases are masked by the unceasing introduction of 

 new and individual features. The student of the 

 human embryo and foetus is impressed not by its 

 recapitulatory behaviour but by the manner in which 

 new features are being intercalated. Such facts favour 

 Huxley's view that the machinery of evolution works 

 in the body of the embryo uninfluenced by adult 

 experience. 



The Use made by Nature of the Capitalistic 

 System. 



Scientific men do not need to be told that capital 

 is needed for the development and improvement of an 

 invention ; capital is as necessary for the progress of 

 a civilisation as for the extension of a business under- 

 taking. Nature discovered very early in the history 

 of the world that capital is needed for evolutionary 

 progress. A breakfast egg represents the capital set 

 aside for the development of a fowl, and during the 

 incubation period the stock of yolk makes possible 

 any form of experiments which the embryonic cells 

 may tend to make. In the higher mammals the 

 capitalistic system has become fluid and elastic — 

 represented by the mother's blood and milk. The 

 placenta and all accessory structures needed for the 

 lodgment of the young in the mother's womb were 

 invented and elaborated by embryonic cells during 

 the incubating stages in the development of lower 

 vertebrates. The simple yolk capitalistic system, 

 evolved and elaborated by the embryonic cells of 

 lower vertebrates, became, in the higher vertebrates, 

 transformed into the elaborate organisation which 

 gives rise to the placenta, thus securing for the young 

 months of free lodging. When we inquire into the 

 nature of the process which gives rise to the placenta 

 we find that it concerns certain embryonic cells which, 

 in the lower vertebrates, proceed to form part of the 

 belly-wall, part of the bowel, and part of the bladder. 

 These same groups of cells in higher mammals have 

 taken on themselves an entirely new purpose. Instead 

 of proceeding to form the parts of the body just 

 mentioned, they give rise to the placenta and mem- 



branes which envelop the embryo. Here we see that 

 embryonic cells and the machinery which regulates 

 their evolutions have inherent in them a power of 

 working out the most intricate inventions and of 

 effecting structural adaptations of the most service- 

 able kind. 



The Genesis of Man's special Structural 

 Features. 



We need not be surprised, seeing how plastic and 

 resourceful the embryonic tissues are, to find most — 

 but not all— of man's characteristic features appear 

 in a modified form as transitional phases in the fcetal. 

 stages of man's nearest allies — the anthropoid apes. 

 Man's outstanding structural peculiarities have been 

 .produced during the embryonic and foetal stages of 

 his evolutionary history ; the corresponding and some- 

 what similar characters which appear in foetal anthro- _ 

 poids become masked in these animals. by the super- 

 addition of coarser animal features which develop as 

 their intra-uterine Ufe closes, and particularly as their 

 adolescent and adult stages are passed. At birth the 

 brain of the baby gorilla is almost as big as that of 

 the human baby; but whereas the period of rapid 

 growth continues in the human brain throughout 

 infancy, the brain oi_ the_gorilla proceeds after bjxiji 

 at a .slow pace. The human brain retains the rapid 

 rate of foetal growth for two years after birth. My 

 friend Prof. L. Bolk^^ of Amsterdam, who has done 

 so much .to_pr0ve- that man's "dfstinctive characters 

 represent a heritage accumulated in the foetal phase of 

 his development, has shown that the downward bend 

 of the front part of the base of the skull, and the_conse-_ 

 quent^ckward position of the face, occur at an early 

 point of development in all mammals. The cranial 

 bend becomes undone and the face thrust forwards as 

 development proceeds in all mammals, save in man, 

 in whom these foetal features are retained until, and 

 throughout, adult life. The nearest approach to the 

 adult human form occurs in the foetal stages of anthro- 

 ^ojd apes. The foetal cranial bend is not a primitive 

 or ancient" character ; it was worked out in foetal 

 life ; never, until the evolution of man took place, 

 4i4^4Jus feature survive to reach an adult stage. 



Let us take another feature — man's hairless skin, 

 and in the case of the white, races its comparative 

 lack 'of _ pigment.^* In the chimpanzee foetus, at the 

 seventh .month of development, the hair is distributed 

 on the body exactly as in a baby at birth ; there is 

 the same long and fine hair on the scalp ; the same 

 smooth skin covered with a short, almost invisible 

 _iiQwn77TJie skin, too, which afterwards becomes deeply 

 pigmented and black in the adult chimpanzee, at this 

 stage is gray, tinged with a trace of brown. At a still 

 younger stage the skin is almost free from pigment. 

 The young of many of the higher primates are bom 

 with fair hair — often tinged with red. Fair hair is a- 

 -Icetal character of primates-which has become pernia-_ 

 jQ.enFjnr_Northcrn Europeans and is found distributed 



" "The Problem <il OrtlioKuathism," Proc. Konin. Akad. van Wetensch. 

 U Amsterdam, 1922, vol. 25, Nos. 7, 8 ; " On the Significance of Supra- 

 orbital Ridges in the Primates," ibid., 1922, vol. 25, Nos. i, 2 ; " On the 

 Character of Morphologcal Modifications in consequence of Affections of 

 the Endocrine Organs," ibid., 1921, vol. 23, No. 9. 



*♦ See my Hcrter I..ectures, " The Evolution of Human Races in the light 

 of the Hormone Theory," Johns Hopkins Hospital Bulletin, 1922, vol. 33, 

 pp. 155, 195 ; also Prof. C. S. Stockard's " Human Types and Growth 

 Reactions," Amer. Journ. Anat., 1923, vol. 31, p. 261. 



