INTRODUCTION 3 



under Domestication/ which appeared only a few years after 

 Spencer's ' Principles of Biology.'' The enormous wealth of facts 

 bearing on heredity which is accumulated in this book is in itself 

 sufificient to show how the gifted author felt himself urged on all 

 sides to consider this extremely difficult and complicated prob- 

 lem. For although Darwin modestly described his theory as 

 a provisional hypothesis, his was, nevertheless, the first compre- 

 hensive attempt to explain all the known phenomena of heredity 

 by a common principle. The theory has so often been discussed 

 and is so well known that a brief account of its substance will 

 suffice here. 



A multicellular organism, whether animal or vegetable, is 

 gradually built up by cell-division : but it is assumed that this 

 method of multiplication is not the only one. Each cell pos- 

 sesses in addition, at each stage of its development, the power 

 of giving off invisible granules or atoms, which, at a later period 

 and under certain conditions, can develop again into cells simi- 

 lar to those from which they originated. Numbers of these 

 ' gemmules ' are being given olT continually from all cells of the 

 body and conveyed into the blood, and thus circulate through 

 the body, finally settling down in some part, principally in 

 those regions in which the development of offspring will take 

 place later on, z.6'., in buds or germ-cells. As gemmules from 

 all the cells of the body are aggregated in these cells, they invest 

 the latter with the power of developing into a new and complete 

 organism. This occurs as follows : — each gemmule reproduces 

 the cell from which it is derived, and the gemmules of the 

 different cells become active in the same order as that in which 

 the corresponding cells followed each other in the ontogeny of 

 the parent. 



The germ is not by any means composed exclusively of 

 gemmules which have been derived from the organism in which 

 they were formed, but consists, at the same time, of a very 

 large number of gemmules which are derived from parents and 

 ancestors even of very remote generations ; and hence a great 

 many more gemmules take part in each case of ontogeny than 

 there are cells formed. Each cell and each part is represented 

 by a great variety of gemmules. A selection must therefore 

 take place, as only one gemmule can form the required cell, and 

 the rest must remain dormant. In this way a number of gem- 

 mules, which have been hitherto dormant, are transferred from 



