33^ SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



the fact that Brooks also looks upon sexual reproduction as the 

 means employed by nature in order to produce variation. The 

 manner in which he supposes that the variability arises is, 

 however, very different from 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 therefore 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 cir- 

 cumstances, 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 bod}^, but as soon as the conditions of 

 life become unfavourable and its functions are disturbed, the 

 cell ' throws off minute particles which are its germs or gem- 

 mules.' 



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 attract- 

 ing 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 

 fertilization, with that part of the ovum which, in the course of 

 development, 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 



