444 weismann's theory of heredity. 



of j>erm-i>lasin, wiiicli, when expanded into ;i reuniting orgauisin, give 

 that organism tlie best elianee in its struggle for existence. And 

 inasiiuu'li as a certain overi)lns of this i)e('nliai" combination of germ- 

 plasm is intrusted to that organism for bequeatliing to the next gen- 

 eration, this to the next, and so on, it follows that natnral selection is 

 all the while conserving tiiat originally pecnhar combination of germ- 

 l)lasm, until it happens to meet with some other marss of germ-plasm 

 by mixing with which it may still further improve upon its original 

 jieculiarity when, of course, natural selection will seize upon this im- 

 provement to perpetuate as in the previous case. So that on the whole 

 we may say that natural selection is ever waiting and watching forsuch 

 combinations of germ-plasm as will give the resulting organisms the 

 best possible chance in their struggle for existence; while at the same 

 time it is remorselessly destroying all those combinations of germ-plasm 

 which are handed over to the custody of organisms not so well fitted to 

 their conditions of life. 



It only remains to add that, according to Weismann's theory in its 

 strictly logical form, combinations of germ-plasm when once effected 

 are so stable that they would never alter except as a result of entering 

 into new combinations. In otiier words, no external influences or in- 

 ternal processes can ever change the hereditary nature of any particular 

 mixture of germ-plasm, save and except its admixture witii some other 

 germ-plasm, which, being of a nature equally stable, goes to unite with 

 the other in equal proportions as regards hereditary character. So that 

 really it would be more correct to say that any given umss of germ-plasm 

 does not change even when it is mixed with some other n)ass — any 

 more, for instance, than a handful of sand can be said to change when 

 it is mixed with a handful of clay. 



Consequently, we arrive at this curious result. No matter how many 

 generations of organisms there may have been, and therefore no matter 

 how many combinations of germ-plasm may have taken ]»lace to give 

 rise to an existing j)opulation, eat^h existing unit of germ-plasm must 

 have remained of the same essential nature of constitution as when it 

 was first started in its immortal career millions of years ago. Or re 

 verting to our illustrtition of sand and clay, the i)articles of each must 

 always remain the same, no matter how many admixtures they may 

 undergo with particles of other materials, such as chalk, slate, etc. 

 Now inasmuch as it is an essential — because a logically necessary — 

 l)art of Weismann's theory to assume such absolute stability or un- 

 changeableness on the part of germ-plasm, the question arises, and has 

 to be met, — What was the origin of those differences of character in the 

 (lift'ercnt germ-i)lasms of multi-cellular organisms which first gave rise, 

 and still continue to give rise, to congenital variations by their mixture 

 one with another '. This important question Weismann answers by 

 supposing that these differences originally arose out of the differences 

 iuthe uni-cellular organisms, which were the ancestors of the primitive 



