442 welsmann's theory of heredity. 



the somatic cells. Tiiruiiif;- now more especially to the germ cells, these 

 are the receptacles of whatWeissmann calls the germ-plasm ; and this it 

 is that which he supposes to differ in kind from all the other constituent 

 elements of the organism. For the germ-plasm he believes to have had 

 its origin in the uni cellular organisms, and to have been handed down 

 from them in one continuous stream through all successive generations 

 of multi-cellular organisms. Thus, for example, suppose we take a cer- 

 tain quantum of germ-plasm as this occurs in any individual organism 

 of to day. A minute portion of this germ plasm, when mixed with a 

 similarly minute portion from another individual, goes to form a new 

 individual. But in doing so only a portion of this minute portion is 

 consumed; the residue is stored up in the germinal cells of this new 

 individual in order to secure that continuity of the germ-plasm which 

 Weismanu assumes as the necessary basis of his whole theory. Fur- 

 thermore, he assumes that this overplus portion of germ-plasm which is 

 so handed over to the custody of the new individual is there capable 

 of growth or multiplication at the expense of the nutrient materials 

 which are supplied to it by the new somaiu which it finds itself located ; 

 while in thus growing or multiplying it faithfully retains its highly 

 complex character, so that in no one minute particular does any part of 

 a many thousand-fold increase differ as to its ancestral characters from 

 that inconceivably small overplus which was first of all intrusted to 

 the embryo by its parents. Tlierefore one might represent the germ- 

 plasm by the metaphor of a yeast-plant, a single particle of which may 

 be put into a vat of nutrient fluid ; there it lives and grows upon the 

 nutriment supplied, so that a new particle may next be taken to impreg- 

 nate another vat, and so on ad infinitum. Here the successive vats 

 would represent successive generations of progeny ; but to make the 

 metaphor complete one would require to suppose that in each case the 

 yeast-cell was required to begin by making its own vat of nutrient 

 material, and that it was only the residual portion of the cell which 

 was afterwards able to grow and multiply. But although the meta- 

 phor is necessarily a clumsy one, it may serve to emphasize the all im- 

 portant feature of Weismanu's theory, viz., the almost absolute inde- 

 pendence of the germ-plasm. For just as the properties of the yeast- 

 plant would be in no way affected by anything that might happen to 

 the vat short of its being broken up or having its malt impaired, so 

 according to Weismann the properties of the germ-plasm cannot be 

 affected by anything that may happen to its containing soma short of 

 the soma being destroyed or having its nutritive functions impaired. 



Such being the relations that are supposed to obtain between the 

 soma and its germ plasm, we have next to contemplate what is sup- 

 posed to happen when, in the course of evolution, some modification of 

 the ancestral form of the soma is required in order to adapt it to some 

 change on the part of its environment. In other words, we have to con- 

 sider Weismann's views on the modus operandi of adaptive develop- 

 ment, with its results in the origination of new species. 



