404: PROGRESS OF SCIENCE IN THE CENTURY. 



to be judged, like all such formulae, by its adequacy 

 in fitting facts. 



Let us suppose that the fertilised ovum has cer- 

 tain qualities, a, h, c, . . , x, y, z; it divides and 

 re-divides, and a body is built up; the cells of this 

 body exhibit division of labour and differentiation, 

 losing their likeness to the ovum and to the first re- 

 sults of its cleavage. In some of the body-cells the 

 qualities a, h, find predominant expression, in others 

 the qualities y, z, and so on. But if, meanwhile, 

 there be certain germ-cells which do not differentiate, 

 which retain the qualities a,h, c, . . . x, y, z, un- 

 altered, which keep up, as one may say figuratively, 

 " the protoplasmic tradition,'' these will be in a posi- 

 tion by and by to develop into an organism like that 

 which bears them. Similar material to start with, 

 similar conditions in which to develop, therefore, 

 like tends to beget like. Various attempts have been 

 made to elaborate the general idea of genetic con- 

 tinuity, in terms for instance of " organic memory " 

 (Haeckel, Hering, Samuel Butler) but it is doubtful 

 whether they have been of real service. 



It may be mentioned that Jaeger, Brooks, De 

 Vries, and others have tried to combine the modern 

 view with a modified version of the pangenetic hy- 

 pothesis. 



(11.) The dual nature of inheritance is another 

 great fact, though it may seem a commonplace to 

 the superficial. x\part from exceptional cases (a- 

 sexual multiplication, parthenogenesis, and autog- 

 amy), the inheritance of every multicellular plant or 

 animal is dual, part of it comes from the mother in 

 the ovum or ovum-nucleus, part of it comes from 

 the father in the spermatozoon or sperm-nucleus ; the 

 beginning of the new individuality is a fertilised 



