ON HEREDITY.* 



1>Y Sir William Txtrnek. 



The subject of heredity (if I iniiy say so) is in the air at the present 

 time. The prominence which it has assumed of late years is in connec- 

 tion with its bearing on the Darwinian theory of natural selection, and 

 consequently biologists generally have had their attention directed to 

 it. But in its relations to man, his structure, functions, and diseases, it 

 has long occupied a prominent position in the minds of anatomists, 

 l)liysiologists, and physicians. That certain diseases, for example, are 

 hereditary was recognized by Uippocrates, who stated generally that 

 hereditary diseases are difficult to remove, and the iniluence which the 

 hereditary transmission of disease exercises upon the duration of life 

 is the subject of a chapter in numerous works on practical medicine, 

 and forms an important element in the valuation of lives for life insur- 

 ance. 



The first aspect of the question which has to be determined is whether 

 any physical basis can be found for heredity. The careful study, es[)e- 

 cially during the last few years, of the development of a number of 

 species of animals, mostly but not exclusively amongst the invertebrata, 

 by various observers, has established the important fact that the young 

 animal arises by the fusion within the egg or germ-cell of an extremely 

 minute particle derived from the male parent, with an almost equally 

 minute particle, derived from the germ cell produced by the female 

 parent. These particles are technically termed in the former case the 

 male pronxdeKfi, in the latter the female promiclens^ and the body formed 

 by their fusion is called the se(jmentaUon nucleus. These nuclei are so 

 small that it seems almost a contradiction in terms to speak of their 

 magnitude, — rather one might say their minitude; for it requires the 

 higher powers of the best microscopes to see them and follow out the 

 l>rocess of conjugation. But notwithstanding their extreme minute- 

 ness, the pronuclei and the segmentation nucleus are complex, both in 

 chemical and molecular structure. From the segmentation nucleus pro- 

 •luced by the fusion of the pronuclei with each other, and from corre- 



'I'lt'sLdential address before tlif Aiitliro[)()I()j;ieal Section of tlio rjritisli Association, 

 A. S., at Newcastlei, September, 1881). {Hqxtrl of the ISritinh AsmnaCtov, vol. Lix, pp. 

 7r.r,-771.) 



