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variations that have occurred in the course of the thousands of 

 years during which sexual reproduction has been in vogue. 



Congenital variations are, in fact, dependent on variations of 

 the germ-plasm out of which organisms are built, and this comes- 

 to the same thing as saying that natural selection works in and 

 through these variations of the germ-plasm — that is to say, 

 through natural selection. For picking out the indiAiduals that 

 are best adapted to their own environment, it is really picking 

 out the most suitable and advantageous combinations of germ- 

 plasms which, when expanded into the resulting organisms, realise 

 the best opportunities for survival in the struggle for existence 

 through which they have to pass. And, as we have already 

 stated, a certain overplus of this germ-plasm, not being used up 

 in the somatic or personal structure of a given organism, is handed 

 over to the keeping of an individual of the next generation, and 

 from this a still smaller fraction proceeds to the next, and so on, 

 preserving all the while its own peculiar characteristics, until in 

 the course of sexual reproduction it meets with another fragment 

 of germ-plasm, which may still further improve its qualities, or 

 the reverse. In the former case the fusion will result in the 

 maturation of an organism still better equipped for the struggle 

 for existence than either parent, and natural selection will seize 

 upon the improvement, and cause it to be perpetuated. 



Putting the matter, then, briefly, we may say that natural 

 selection is ever waiting and watching for such fresh and fortui- 

 tous combinations of germ-plasm as will produce individuals best 

 calculated for survival, and on the other hand that destruction 

 lies in wait for those combinations which result in organisms less- 

 favourably endowed. 



A stability on the part of germ-plasm, unchangeable and un- 

 alterable except by the modifying influence of other germ-plasma 

 itself equally stable, stands, then, as an essential feature of Weis- 

 mann's theory. The germ-plasm of any organism being of the 

 same essential nature as when it started on its career thousands 

 of years ago. Like the unicellular organisms, it is endowed with 

 the property of everlasting life ; it is immortal. 



Such a conclusion, however, inevitably leads to the question, 

 What was the cause of those aboriginal differences of character 

 in the germ- plasmas of different multicellular organisms which 

 first gave rise to congenital variations ? 



Weismann answers that those differences arose out of the 

 original diflerences in unicellular organisms which were the ances- 

 tors of the multicellular. The former differ from the latter, as 

 we have seen, in that they alone can be influenced by the action 

 their environment and the different species ot unicellular organisms 

 arose in consequence of these differences of reaction. 



