EVOLUTION OF THE COLOKS OP BIRDS. 15 



tained — when no longer necessary?" The above pas- 

 sage is open to two objections. First, he assumes pan- 

 mixia as proved. In discussing this subject later on it 

 will be shown that, on the contrary, panmixia appears 

 to be largely untenable. Secondly, panmixia means a 

 cessation of natural selection. If we assume with Weis- 

 mann, as there seems every reason to assume, that the 

 original unicellular organisms, and their living repre- 

 sentatives to-day, possess a potential immortality or 

 possibility of indefinite existence, we certainly cannot 

 assume that some are more immortal than others. But 

 if potential immortality be a natural attribute of life, 

 why should natural selection be necessary to preserve 

 this attribute, or from what could it make its selection? 

 If natural selection is not requisite to maintain this 

 standard of immortality, panmixia, assuming its po- 

 tency in other instances, could have no influence in 

 causing mortality, being merely the negative of natural 

 selection. 



Prof. Weismann then replies to Prof. Vine's criticism 

 of his theory of embryogenesis and the continuity of 

 germ-plasm. He asserts that Prof. Vine's criticism is 

 due to a misconception , that he does not claim that germ- 

 plasm is ever converted into somatoplasm. In his second 

 essay he had indeed contrasted the somatoplasm or the 

 entire substance of the body with the germ-plasm or 

 entire substance of the germ-cells, not having arrived at 

 the time at the conclusions of Strasburger and 0. Hert- 

 wig, that hereditary transmission was effected solely by 

 the chromatin of the nuclear loops. This view he had 

 adopted when the fourth essay was written, and his theory 

 was accordingly somewhat modified. He made use of Na- 

 geli'sterm, idioplasm, in an essentially different manner, 

 applying it to the chromatin not only of the ovum- 

 nucleus, but also of every cell in the body. This idio- 



