In the last two decades of the nineteenth century much that is new has been introduced into biological science; Nägeli's idea of 'idioplasm'--the substance which determines form; Roux's _Struggle of the Parts_, the recognition of a special hereditary substance, 'the germ-plasm,' its analysis into chromosomes, 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 mingling 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 and writings of others as possible without making the book heavy; but my aim has been to write a book to be read, not merely one to be referred to. If it be asked, finally, for whom the book is intended, I can hardly answer otherwise than 'For him whom it interests.' The lectures were delivered to an audience consisting for the most part of students of medicine and natural science, but including some from other faculties, and sometimes even some of my colleagues in other departments. In writing the book I have presupposed as little special knowledge as possible, and I venture to hope that any one who _reads_ the book and does not merely skim it, will be able without difficulty to enter into the abstruse questions treated of in the later lectures.