author's preface 



In the last two decades of the nineteenth centu 

 that is new has been introduced into biological 

 Nageli's idea of ' idioplasm ' — the substance which de- 

 form ; Eoux's Struggle of the Parts, the recognition of 

 hereditary substance, ' the germ-plasm,' its analysis ii 

 mosomes, and its continuity from generation to ger 

 the potential immortality of unicellular organisms ar 

 germ-cells in contrast to the natural death of higher f( 

 ' bodies ' ; a deeper interpretation of mitotic nuclear 

 the discovery of the centrosphere — the marvellous 

 apparatus of the cell — which at once allowed us to \ 

 a whole stratum deeper into the unfathomable 

 microscopic vital structure ; then the clearing up of ( 

 in regard to fertilization, and the analysis of this into 

 processes combined in it, reproduction and the mil 

 the germ-plasms (Amphimixis) ; in connexion with 

 phenomena of maturation, first in the female and the 

 male cell, and their significance as a reduction of the h( 

 units : — all this and much more we have gained du 

 period. Finally, there is the refutation of the Lan 

 jDrinciple, and the consequent elaboration of the ] 

 of selection by applying it to the hitherto closed r 

 the ultimate vital elements of the germ-plasm. 



The actual form of these lectures has developed 

 were transcribed. But although the form is thus 

 extent new, I have followed in the main the sar 

 of thought as in the lectures of recent years. The 

 form has been adhered to in the book, not merely 

 of the greater vividness of presentation which it 

 but for many other reasons, of which the greater 

 in the choice of material and the limiting of quot 

 a minimum are not the least. That all polemics of a 

 kind have thus been excluded will not injure the b 

 it is by no means lacking in discussions of opinion, g 

 therefore, I trust, contribute something towards the 

 up of disputed points. 



I have endeavoured to introduce as much of the re 



