MAN AND MANKIND 



cancer by a derangement of the nucleic acid cycle. The mechanism 

 is like that found with the development of pernicious anaemia, 

 which leads to upset of mitosis and the formation of both polyploid 

 and sub-diploid nuclei. The defective nuclei survive through existing 

 in groups whose members, having no differentiated cells or spheres 

 of action, support one another. Sufficient breakage of chromosomes 

 by irradiation, however, can give rise to defective nuclei which are 

 unable to support one another. A tumour whose growth has 

 presumably been started by the cytoplasm can thus be stopped by 

 the nucleus. Finally, it has been possible to show in mice as well as 

 in man that cancer is determined by heredity, although the pene- 

 trance of the genotype is often low, the growth of a tumour 

 being subject to environmental conditioning. At every point of 

 causation, diagnosis, and treatment, the techniques of genetics or 

 cytology are thus concerned with cancer. 



The fourth respect in which human heredity is pre-eminent is 

 that in which it is obviously unique, namely in the study of the 

 mind, or of the body by way of the mind. The simple properties of 

 taste and allergy, which must obviously be hereditary in all animals, 

 can be readily shown to be so only in man: the ape can merely 

 confirm what the man has discovered. And when we come to the 

 still more delicate question of instincts, it is only in man and his 

 domesticated dog that the individual character shows the hereditary 

 diversity of the species. 



These foundations are important for their direct assistance to 

 education and medicine, but they are still more important for their 

 potential assistance to research. They provide us with a programme 

 of future research on a broader basis. They show the importance, 

 for diagnosis and for treatment, of accurate medical recording of 

 family histories, and especially of those of identical twins. They 

 show that the extended study of blood groups is bound to yield 

 fruit of value in three different fields. They show that our practical 

 knowledge of the relations of host to disease, gained in agriculture, 

 can be applied with advantage to the study of immunity and 

 epidemics in man. They show that the chromosomes can be used 

 in the study of human development and disease with a profit that 

 can as yet hardly be estimated. 



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