350 Professor J. Arthur Thomson [March 30, 



molecular structure which is the bearer of the hereditary qualities. 

 In development a part of the germ-plasm, " contained in the parent 

 egg-cell, is not used up in the construction of the body of the off- 

 spring, but is reserved unchanged for the formation of the germ-cells 

 of the following generation." Thus the parent is rather the trustee 

 of the germ-plasm than the producer of the child. In a new sense, 

 the child is a chip of the old block. Early segregation of the germ- 

 cells is in many cases an observable fact — and doubtless the list of 

 such cases will be added to — the conception of a germ-plasm is 

 hypothetical, just as the conception of a specific living stuff or 

 protoplasm is hypothetical. In the complex microcosm of the cell, 

 we cannot point to any one stuff and say " this is protoplasm " ; it 

 may well be that vital activity depends upon several complex stuffs 

 which, like the members of a carefully constituted firmware charac- 

 teristically powerful only in their inter-relations. In the same way, 

 it must be clearly understood that we cannot demonstrate the germ- 

 plasm, even if we may assume that it has its physical basis in the 

 stainable nuclear bodies or chromosomes. The theory has to be 

 judged, like all conceptual formulas, by its adequacy in fitting facts. 



Let us suppose that the fertilised ovum has certain qualities, 

 a, b, 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 results of its cleavage. 

 In some of the body-cells the qualities a, b find predominant expres- 

 sion, 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, b, c,...x, y, z, unaltered, which keep up, as one may 

 say figuratively, " the protoplasmic tradition," these will be in a 

 position 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. 



May we think for a moment of a baker who has a very precious 

 kind of leaven, and some less precious material on which this may 

 work ; he uses both in baking a large loaf; but he so arranges matters 

 by a clever contrivance that part of the original leaven is always 

 carried on unaltered, carefully preserved for the next baking. Nature 

 is the baker, the loaf is a body, the leaven is the germ-plasm, and each 

 baking is a generation. 



II. Dual Nature of Inheritance. 



Apart from exceptional cases, the inheritance of a multicellular 

 animal or plant is dual, part of it comes from the mother and part 

 of it from the father ; the beginning of the new individuality is a 

 fertilised egg-cell. The exceptions referred to are cases of asexual 

 multiplication by buds or otherwise, as in the freshwater Hydra ; 

 cases of parthenogenesis, as in the case of the urifertilised eggs which 



