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.
It would be a great satisfaction to me if this book were to be the means of introducing my theoretical views more freely among investigators, and to this end I have elaborated special sections more fully than in the lectures. Notwithstanding much controversy, I still regard its fundamental features as correct, especially the assumption of 'controlling' vital units, the determinants, and their aggregation into 'ids'; but the determinant theory also implies germinal selection, and without it the whole idea of the guiding of the course of transformation of the forms of life, through selection which rejects the unfit and favours the more fit, is, to my mind, a mere torso, or a tree without roots.
I only know of two prominent workers of our day who have given thorough-going adherence to my views: Emery in Bologna and J. Arthur Thomson in Aberdeen. But I still hope to be able to convince many others when the consistency and the far-reachingness of these ideas are better understood. In many details I may have made mistakes which the investigations of the future will correct, but as far as the basis of my theory is concerned I am confident: the principle of selection does rule over all the categories of vital units. It does not, indeed, create primary variations, but it determines the paths of evolution which these are to follow, and thus controls all differentiation, all ascent of organization, and ultimately the whole course of organic evolution on the earth, for everything about living beings depends upon adaptation, though not on adaptation in the sense in which Darwin used the word.
The great prominence thus given to the idea of selection has been condemned as one-sided and exaggerated, but the physicist is quite as open to the same reproach when he thinks of gravity as operative not on our earth alone, but as dominating the whole cosmos, whether visible to us or not. If there is gravity at all it must prevail everywhere, that is, wherever material masses exist; and in the same way the co-operation of certain conditions with certain primary vital forces must call forth the same process of selection wherever living beings exist; thus not only are the vital units which we can perceive, such as individuals and cells, subject to selection, but those units the existence of which we can only deduce theoretically, because they are too minute for our microscopes, are subject to it likewise.
This extension of the principle of selection to all grades of vital units is the characteristic feature of my theories; it is to this idea that these lectures lead, and it is this—in my own opinion—which gives this book its importance. This idea will endure even if everything else in the book should prove transient.
Many may wonder, perhaps, why in the earlier lectures much that has long been known should be presented afresh, but I regard it as indispensable that the student who wishes to make up his own mind in regard to the selection-idea should not only be clear as to what it means theoretically, but should also form for himself a conception of its sphere of influence. Many prejudiced utterances in regard to 'Natural Selection' would never have been published if those responsible for them had known more of the facts; if they had had any idea of the inexhaustible wealth of phenomena which can only be interpreted in the light of this principle, in as far, that is, as we are able to give explanations of life at all. For this reason I have gone into the subject of colour-adaptations, and especially into that of mimicry, in great detail; I wished to give the reader a firm foundation of fact from which he could select what suited him when he wished to test by the light of facts the more difficult problems discussed in the book.
In conclusion, I wish to thank all those who have given me assistance in one way or other in this work: my former assistant and friend Professor V. Häcker in Stuttgart, my pupils and fellow workers Dr. Gunther and Dr. Petrunkewitsch, and the publisher, who has met my wishes in the most amiable manner.
Freiburg-I-Br.,
February 20, 1902.