Stipplement to ''Nature'' August i8, 1923 



265 



support the jaws ; so were the muscles of mastication, 

 the temporo-mandibular joint — in short, all dental, 

 bony, muscular, vascular, and nervous structures con- 

 cerned in mastication. We cannot conceive how such 

 disorders of growth could be so sharply limited to a 

 single functional system unless we agree that the 

 machinery which regulates growth and development 

 is organised not on an anatomical, but on a functional 

 basis. 



Use-inheritance. 



In the foregoing paragraphs an attempt has been 

 made to picture the means by which the development 

 and growth of the various cell groups, which make up 

 the body of the embryo, are co-ordinated and controlled. 

 Such evidence as we have justifies us in the belief that 

 there is an automatic system of control worked by 

 means of hormones, and that this machinery, in all its 

 variations, tends to produce a functional or adapta- 

 tional result. The very important question remains 

 to be considered : can this machinery, which controls 

 the differentiation of the tissues of an embryo, be 

 influenced from without ? Or does it, as Bateson 

 l)elieves, work on towards its destined result, in spite 

 of all surrounding conditions and influences ? The 

 uenital glands and their contents, of both man and 

 woman, are exposed to all the substances, be they 

 nutritive or hormonic in nature, which flood their 

 circulatory systems. In 1906 J. J. Cunningham ^^ 

 applied the theory of hormones to the problems of 

 heredity. He conceived it possible that the genital 

 cells could be influenced, and so altered in their con- 

 stitution, by hormones thrown ofT l^y all the organs 

 and parts of the parent body. There is no inherent 

 physical obstacle to prevent one from entertaining 

 such a belief. Such a conception implies the possi- 

 bihty of hormones — function-regulating substances — ■ 

 of a parent coming into contact with and influencing 

 the controlling action of the embryonic hormone- 

 system. If it were possible, as is assumed in every 

 form of Lamarckian lielief, for parent products to come 

 in contact with, and thus alter, the machinery' which 

 controls the growth of the embryo, it would be a con- 

 sequence of the utmost import for mankind. By a 

 full use of our brains, of our teeth, or of our hands, we 

 might hope to influence the development and growth 

 of the corresponding parts in our children. 



Evidence of the Teeth. 



I have selected the teeth to test the question as to 

 the part played by use in the evolution of structural 

 adaptations. There can be no doubt that the manner 

 jn which the crowns of man's sixteen upper teeth fit 

 against corresponding surfaces of the lower sixteen, 

 give us as fine a structural adaptation as we may hope 

 to cite. There is the additional advantage that, as 

 the teeth are the most persistent of fossil remains, we 

 know more of this system in the forerunners of man 

 and of living anthropoid apes than of any other parts 

 of their anatomy. Further, in highly civilised races 

 teeth are not only more liable to decay and to irregu- 

 larities of eruption than in primitive races, but there 

 is also, in civilised peoples, a marked tendency to a 

 reduction in size and number of the dental series. We 



" Hormones an J Heredity, 1921. 



see, too, in the evolution of the dentitions of the higher 

 primates, when the pattern of the enamel changes in 

 one tooth, it changes in all of them ; if one tooth alters, 

 the opposing teeth have to alter in conformity ; we 

 see that if the dentition strengthens, all the members 

 of the series participate ; when reduction sets in, all 

 the teeth suffer a reduction in a definite order. But 

 these changes cannot be due to use, for the crowns of 

 the teeth are laid down, and the opposing chewing 

 surfaces fully formed, while the dental germs lie buried 

 in the gums and long before the crowns come into use. 

 When they do come into use, the teeth formed in the 

 upper jaw possess the exact surfaces needed to oppose 

 those of the lower jaw. After usage, especially in apes 

 and primitive man, the opposing surfaces become 

 worn off ; if use had any effect here it would be to 

 produce teeth with eroded crowns. 



It is clear that functional adaptation, so far as 

 concerns the production of teeth, is a property resident 

 in the embryonic tissues ; the effects of usage in the 

 parent can have no influence on the machinery which 

 shapes the dental crowns in the mouth of the foetus 

 and infant. If this is true of one system of the human 

 body, it is probably true of all other adaptational 

 systems— such as the brain, hand, and foot. Nature 

 would have been foolhardy to entrust the future of 

 any race whatsoever to the voluntary efforts or natural 

 inclinations of the parents. As far as possible she 

 seems to have safeguarded the progeny by isolating 

 the gonads from the functional influences of the 

 parental body. 



The Germ-plasm can be permanently injured. 



Yet there is one line of evidence which shows that 

 the spermatozoa of the male and the ova of the female 

 can be acted on or injured from without. Darwin 2* 

 has related the case of a cow in which one eye was 

 injured when she was in calf. The calf was born with 

 the corresponding eye small and blind. In more 

 recent years Marey ^^ has recorded an identical result 

 in a mare ; one eye was injured when she was pregnant, 

 and the foal was born with the corresponding eye 

 small and blind. Hitherto we have been inclined to 

 regard such cases as mere coincidences, but the well- 

 known experiments of Guyer and Smith ^^ provide a 

 rational explanation. They injected into the veins 

 of doe rabbits, about the end of the second week of 

 pregnancy, doses of a substance which has a selective 

 and toxic action on the lens of the eye. Many of the 

 young were l^orn with defects of the eyes — cataract of 

 the lens ]:)eing particularly frequent. When these 

 young ralibits grew up and bred, many of their young 

 showed the same defects. The developmental dis- 

 order could be transmitted in the spermatozoa as well 

 as in the ova. These experiments show that the 

 germ-plasm can be reached from without, and by 

 means of a toxic substance can be permanently injured, 

 so that progeny issuing from it will show ever afterwards 

 a characteristic and localised defect. Prof. Ch. R.. 



** Variations in Plants and Animals under Domestication, 1868, vol. 2, 



P- 34. 



" Le D6terminismc et I'adaptatiou morphologique, R. Anthony, 192a, 

 p. 88. 



»• M. V. Guyer and E. A. Smith, Journ. Experim. Zoology, 1921, vol. 31, 

 p. 171. 



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