PREFACE XI 



the principal data bearing upon the question. I have myself 

 more than once abandoned a line of research undertaken in 

 connection with the problem of heredity, because I felt that 

 to proceed without the guidance of a theory more or less 

 complete in itself, and developed on a basis of ascertained 

 facts, would be little better than groping in the dark. The 

 importance of such a theory lies primarily in its suggestive- 

 ness, by which alone it becomes a step towards the ideal at 

 which we aim, viz., the formulation of the true and co?nplete 

 theory. 



The growth of this book has been very gradual. What 

 first struck me when I began seriously to consider the 

 problem of heredity, some ten years ago, was the necessity 

 for assuming the existence of a special organised and living 

 hereditai-y substance, which in all multicellular organisms, 

 unlike the substance composing the perishable body of 

 the individual, is transmitted from generation to generation. 

 This is the theory of the contimtity of the germ-plasm. My 

 conclusions led me to doubt the usually accepted view 

 of the transmission of va7'iations acqiiired by the body 

 (soma) ; and further research, combined with experiments, 

 tended more and more to strengthen my conviction that in 

 point of fact no such transmission occurs. Meanwhile, the 

 investigations of several distinguished biologists — in which I 

 myself have had some share — on the process of fertilisation 

 and conjugation, brought about a complete revolution in 

 our previous ideas as to the meaning of this process, and 

 further led me to see that the germ-plasm is composed of 

 vital units, each of equal value, but differing in character, 

 containing all the primary constituents of an individual. 

 These ' ancestral germ-plasms ' (' Ahnenplasmen '), or ' ids,' as 

 I now prefer to call them, afforded additional matter where- 



