760 REPOET— 1889. 



To make these somewhat abstract propositions a little more clear, I have de- 

 vised the following graphic mode of representation : — 



A T^ r^ "n 



yT\ j ab^ \abc/ Lbcd/ 



Let the capital letters A, B, C, D, &c., express a series of successive generations. 

 Suppose A to be the starting-point, and to represent the somatic or personal struc- 

 ture of an individual ; then a may stand for the reproductive cells, or germ-plasm, 

 from which the offspring of A, viz. B, is produced. B, like A, has both a personal 

 structure and reproductive cells or germ-plasm, the latter of which is represented 

 by the letters ah, which are intended to show that whilst belonging to B they 

 have a line of continuity with A. stands for an individual of the third genera- 

 tion, ia which the reproductive plasm is indicated by adc, to express that, though 

 within the body of C, the germ-plasm is continuous with that of both b and a. D 

 also contains the reproductive cells abed, which are continuous with the germ- 

 plasm of the three preceding generations, and so on. 



It follows, therefore, from this theory that the germ-plasm possesses throughout 

 the same complex chemical and molecular structure, and that it would pass through 

 the same stages when the conditions of development are the same, so that the same 

 final product would arise. Each successive generation would have therefore an 

 identical starting-point, so that an identical product would arise from all of them. 



Weismann does not absolutely assert that an organism cannot exercise a modi- 

 fying influence upon the germ-cells within it ; yet he limits this influence to such 

 slight effect as that which would arise from the nutrition and growth of the indi- 

 Tidual, and the reaction of the germ-cell upon changes of nutrition caused by 

 alteration in growth at the periphery, leading to some change in the size, number, 

 and arrangements of its molecular units. But he throws great doubt upon the 

 existence of such a reaction, and he, more emphatically than Mr. Gal ton, argues 

 against the idea that the cells which make up the somatic or personal structure of 

 the individual exercise any influence on the reproductive cells. From his point of 

 view the structural or other properties which characterise a family, a race, or a 

 species are derived solely from the reproductive cells through continuity of their 

 germ-plasm, and are not liable to modification by the action on them of the organs 

 or tissues of the body of the individual organism in which they are situated. To 

 return for one moment to my graphic illustration in elucidation of this part of the 

 theory. The cells which make up the personal structure of A or B would exercise 

 no effect upon the character of the reproductive cells a or ab contained within them. 

 These latter would not be modified or changed in their properties by the action of 

 the individual organism A or B. The individual B would be in hereditary descent, 

 not from A + ci, but only from a, with which its germ-plasma ab would be con- 

 tinuous, and through, which the properties of the family, race, or species would be 

 transmitted to C, and so on to other successive generations. 



The central idea of Heredity is permanency ; that like begets like, or, as Mr. 

 Galton more fitly puts it, that * like tends to produce like.' But though the off- 

 spring conform with their parents in all their main characteristics, yet, as everyone 

 knows, the child is not absolutely like its parents, but possesses its own character, 

 its own individuality. It is easy for anyone to recognise that differences exist 

 amongst men when he compares one individual with another; but it is equally easy 

 for those who make a special study of animals to recognise individual differences in 

 them also. Thus a pigeon or canary fancier distinguishes without fail the various 

 birds in his flock, and a shepherd knows every sheep imder his charge. But the 

 anatomist tells us that these difl^erences are more than superficial — that they also 

 pervade the internal structure of the body. In a paper which I read to the meeting 

 of this Association in Birmingham so long ago as 1865,' after relating a series of 

 instances of variation in structure observed in the dissections of a number of human 



' Transactio?is of Sections, p. Ill, 1865, and Tram. Roy. Soc. Edinburgh, vol. 

 xxiv., 1866. 



