Supplement to "■Nature,'' August j8, 1923 



259 



deserves to be considered here. It was given by 

 Prof. G. Elliot Smith/" and is founded on a prolonged 

 and intimate study of the brain of man and of the 

 brains of animals which have a close structural relation- 

 ship to man : 



"And if all the factors in his (man's) emergence are 

 not yet known, there is one unquestionable, tangible 

 factor that we can seize hold of and examine — the 

 steady and uniform development of the brain along 

 a well-defined course throughout the Primates right 

 up to man, which must give us the fundamental 

 reason for man's emergence and ascent. . . . Thus 

 at the dawn of the Tertiary period there were 

 developed the germs of all the psychical greatness 

 which, in the million or so of years that have followed, 

 culminated in the human mind." 



Without a doubt the brain of the great anthropoids 

 is but an elaborated edition of that which serves the 

 needs of monkeys, and, in turn, the brain of man, 

 while framed on exactly the same plan as that of the 

 great anthropoid, far transcends it in complexity of 

 elaboration. In the evolution of these three stages, 

 represented by the brains of man, anthropoid, and 

 monkey, we are witnessing, not an unpacking, but an 

 ever-increasing degree of speciahsation as von Baer 

 and Spencer recognised long ago. In the organisation 

 of the brain of the monkey we see something which is 

 comparable to the civilisation of a primitive people, 

 such as the aborigines of Australia ; in that of the 

 anthropoids, one which may be compared with the 

 life led by a semi-civihsed people, such as the natives 

 of the Congo, while in the human brain we reach a 

 stage of complexity represented by the highest modern 

 civihsation. Whether we speak of brains or of civilisa- 

 tion, the machinery of evolution must be of an analogous 

 nature in both of them. What is the nature of this 

 machinery ? 



How Adaptations appear during the 

 Development of the Embryo. 



Since the time of Darwin and of Huxley our know- 

 ledge of the factors which take a part in controlling 

 ihe development, and therefore the evolution, of the 

 brain and of its appended sense organs, such as the 

 eye, the ear, and the nose, has entered a new phase. 

 We shall take the formation of the eye as our first 

 example because in design and execution it far excels 

 ;iny camera yet invented ; it has been the theme of 

 many a teleological sermon, and a consideration of its 

 development will take us right to the heart of our 

 subject — the origin of purposive or adapted structures. 

 After the publication of the " Origin of Species," 

 Mr. J. J. Murphy, of Belfast, cited the eye as a structure 

 which could not be accounted for by any theory of 



'• British Association Reports, 1912 (Dundee), pp. 575-598, ' 



selection then propounded. "It is probably no 

 exaggeration to suppose," wrote Mr. Murphy, 



that in order to improve such an organ as the eye 

 at all, it must be improved in ten different ways at 

 once, and the improbability of any complex organ 

 being produced and brought to perfection in any 

 such way is an improbability of the same kind and 

 degree as that of producing a poem or a mathematical 

 demonstration by throwing letters at random on a 

 table." 



Darwin, with that customary candour which regu- 

 lated his search for truth, quotes in full this cogent 

 and, to my way of thinking, just criticism, and Darwin's 

 reply was that the eyes of men, as of animals, did show 

 slight degrees of individual variation, and that he 

 could conceive the twilight eye of the owl or of the 

 lemur as having arisen by a selection and accumulation 

 of these minute variations. Mr. Murphy modestly 

 estimated the parts of the eye which must undergo a 

 simultaneous modification, if sight was to remain 

 efficient, as ten in number ; he would have been inside 

 the mark if he had said ten thousand. We cannot 

 conceive how the countless elements which go to the 

 construction of an eye can assume their appropriate 

 place, form, and function unless we postulate a 

 machinery which regulates the development and growth 

 of every one of them. 



The existence of such a machinery was made evident 

 by experiments on tadpoles carried out by Dr. Warren 

 H. Lewis at Baltimore from 1903 onwards. ^^ The 

 optic cup, which ultimately forms the retina of the 

 eye, grows out from the wall of the brain towards the 

 embryonic skin or ectoderm. When this cup comes 

 into contact with the ectoderm, the overlying cells 

 begin to proliferate and arrange themselves so as to 

 form a transparent or crystalline lens. Dr. Lewis 

 transplanted the outgrowing optic cups of tadpoles, and 

 found, if they were placed under the ectoderm of the 

 neck or of the belly, that the result .was the same ; an 

 optic cup caused the overlying cutaneous cells to alter 

 their nature and form a lens. Dr. Lewis realised the 

 significance of his discovery ; in the developing embryo, 

 although only of certain species, one group of living 

 cells can enslave and control the behaviour of another 

 group. He gave us a glimpse of the kind of evolu- 

 tionary machinery employed in fashioning a highly 

 purposive structure such as the eye. Any one who 

 has followed the success with which physicists have un- 

 ravelled the structure of the atom in recent years will 

 not despair of an equal success attending the efforts of 

 embryologists to uncover the means by which one 



" The Variation of Animals and' Plants under Domestication, 1868, 

 vol. 2, p. 222. 



" " Experiments on the Origin and DilTerentiation of the Optic Vesicle 

 of Amphibia," Amer. Jour of Anal., 1904, vol. 3, pp. 507, 805 ; 1907, vol. 7, 

 pp. 144, 259. See also Spcmann, Zoolog. Jahrbuch, 1912, vol. 32, p. i. 



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