Weisrnannism up to date (1893). *49 



progeny — when the process of natural selection was 

 immediately inaugurated, and thereafter entirely 

 superseded the Lamarckian factors. Or, to state the 

 matter in Weismann's own words : — 



My earlier views on unicellular organisms as the source of 

 individual differences, in the sense that each change called forth 

 in them by external influences, or by use and disuse, was 

 supposed to be hereditary, must therefore be dismissed to some 

 stage less distant from the origin of life. I now believe that 

 such reactions under external influences can only obtain in the 

 lowest organisms which are without any distinction between 

 nucleus and cell-body. All variations which have arisen in 

 them, by the operation of any causes whatever, must be in- 

 herited, and their hereditary individual variability is due to the 



direct influence of the external world If I am correct in 



my view of the meaning of conjugation as a method of amphi- 

 mixis, we must believe that all unicellular organisms possess it, 

 and that it will be found in numerous low organisms, in which 

 it has not yet been observed 1 . 



It is not very clear, at first sight, how Professor 

 Weismann, after having thus abandoned the pro- 

 positions 1 , 1, and 3, as above stated, manages to retain 

 his former view as given in No. 4. Nevertheless he 

 does so, by representing that a unicellular organism, 

 even though it present such a considerable degree of 

 organization as we meet with in the higher Protozoa, 

 still resembles a germ-cell of a multicellular organism, 

 in that it consists of all the essential constituents of 

 a germ-cell, including germ-plasm in its nucleus. And 

 inasmuch as a germ-cell is potentially immortal, so it 

 must be with a unicellullar organism ; in the one 

 case, as in the other, the design of the structure is 

 that its contained germ-plasm shall fuse with the germ- 

 plasm contained in the nucleus of another individual 



1 Essays on Heredity, vol. ii. pp. 193-4. 



