AUTHOR S PREFACE VU 



In the last two decades of the nineteenth century much 

 that is new has been introduced into biological science ; 

 Nageli's idea of ' idioplasm ' — the substance which determines 

 form ; Koux's Struggle of the Parts, the recognition of a special 

 hereditary substance, ' the germ-plasm,' its analysis into chro- 

 mosomes, and its continuity from generation to generation ; 

 the potential immortality of unicellular organisms and of the 

 germ-cells in contrast to the natural death of higher forms and 

 ' bodies ' ; a deeper interpretation of mitotic nuclear division, 

 the discovery of the centrosphere — the marvellous dividing 

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

 a whole stratum deeper into the unfathomable mine of 

 microscopic vital structure ; then the clearing up of our ideas 

 in regard to fertilization, and the analysis of this into the two 

 processes combined in it, reproduction and the mingHng of 

 the germ-plasms (Amphimixis) ; in connexion with this, the 

 phenomena of maturation, first in the female and then in the 

 male cell, and their significance as a reduction of the hereditary 

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

 period. Finally, there is the refutation of the Lamarckian 

 principle, and the consequent elaboration of the principle 

 of selection by applying it to the hitherto closed region of 

 the ultimate vital elements of the germ-plasm. 



The actual form of these lectures has developed as they 

 were transcribed. But although the form is thus to some 

 extent new, I have followed in the main the same train 

 of thought as in the lectures of recent years. The lecture- 

 form has been adhered to in the book, not merely because 

 of the greater vividness of presentation which it implies, 

 but for many other reasons, of which the greater freedom 

 in the choice of material and the limiting of quotation to 

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

 kind have thus been excluded will not injure the book, but 

 it is by no means lacking in discussions of opinion, and will, 

 therefore, I trust, contribute something towards the clearing 

 up of disputed points. 



I have endeavoured to introduce as much of the researches 



