IN THE THEORY OP NATURAL SELECTION. 327 



that suggested in my theory, and our fundamental conceptions are 

 also widely divergent. "While I look upon the continuity of the 

 germ-plasm as the foundation of my theory of heredity, and there- 

 fore believe that permanent hereditary variability can only have 

 arisen through some direct change in the germ-plasm effected by 

 external influences, or following from the varied combinations which 

 are due to the mixture of two individually distinct germ-plasms 

 at each act of fertilization, Brooks, on the other hand, bases his 

 theory upon the transmission of acquired characters, and upon the 

 idea which I have previously called 'the cyclical development of 

 the germ-plasm.' 



Brooks' theory of heredity is a modification of Darwin's pan- 

 genesis, for Brooks also assumes that minute gemmules are thrown 

 off by each cell in the body of the higher organisms ; but such 

 gemmules are not emitted always, and under all circumstances, 

 but only when the cell is subjected to unaccustomed conditions. 

 During the persistence of the ordinary conditions to which it is 

 adapted, the cell continues to perform its special functions as part 

 of the body, but as soon as the conditions of life become unfavour- 

 able and its functions are disturbed, the cell ' throws off minute 

 particles which are its germs or gemmules.' 



These gemmules may then pass into any part of the organism ; 

 they may penetrate the ova in the ovary, or may enter into a bud, 

 but the male germ-cells possess a special power of attracting them 

 and of storing them up within themselves. 



According to Brooks, variability arises as a consequence of the 

 fact that each gemmule of the sperm-cell unites, during fertiliza- 

 tion, with that part of the ovum which, in the course of develop- 

 ment, is destined to become a cell corresponding to that from 

 which the gemmule has been derived. 



Now, when this cell developes in the offspring, it must, as a 

 hybrid, have a tendency to vary. The ova themselves, as cells, 

 are subject to the same laws ; and the cells of the organism will 

 continue to vary until one of the variations is made use of by 

 natural selection. As soon as this is the case, the organism 

 becomes, ijpso facto, adapted to its conditions ; and the production 

 of gemmules ceases, and with it the manifestation of variability 

 itself, for the cells of the organism then derive the whole of their 

 qualities from the egg, and being no longer hybrid, have no 



