446 weismann's theoky of heredity. 



iu the case of all the iini-cellular organisms. But inasmuch as the nml- 

 ti-cellular organisms were all iu the first instance derived from the uni- 

 cellular and inasmuch as their germ-plasm is of so stable a nature that 

 it can never be altered by any agencies internal or external to the 

 organisms i^resenting it, it follows that all congenital variations are 

 the remote consequences of aboriginal differences on the part of uni- 

 cellular ancestors. And lastly, it follows also that these congenital 

 variations — although uow so entirely independent of external conditions 

 of life, and even of activities internal to organisms themselves — were 

 originally and exclusively due to the direct action of such conditions 

 on the lives of their unicellular ancestry; while even at the present 

 day no one congenital variation can arise which is not ultimately due to 

 difterences impressed upon the protoplasmic substance of the germinal 

 elements, when the parts of which these are uow composed constituted 

 integral parts of the protozoa, which were directly and differentially 

 affected by their couv^erse with their several environments. 



Such then is Weismann's theory of hei*edity in its original and 

 strictly logical form. But it is now necessary to add that in almost 

 every one of its essential features, as just stated, the theory has had to 

 undergo — or is demonstrably destined to undergo — some radical modi- 

 fication. On the present occasion however, my object is merely to 

 state the theory, not to criticise it. Therefore I have sought to present 

 the whole theory in its completely connected shape. On a future occa- 

 sion — I hope within the present year — it will be my endeavor to dis- 

 connect the now untenable parts from the parts which still remain for 

 investigation at the hands of biological science. 



