REPR OD UCTION. 941 



one of these various units is possessed of the fundamental vital properties of 

 growth and multiplication by division. Such a complex system is Preforma- 

 tion in an extreme form. In fertilization idants of the sperm join with idants 

 of the ovum, and the resulting segmentation nucleus consists of a mixture of 

 paternal and maternal determinants. Within this mixture there exist in a 

 potential state the primary constituents of a considerable number of forms 

 which the future individual may assume. In ontogeny, or development of 

 the individual, these primary constituents take two paths : some of the ids 

 remain inactive and enter the germ-cells of the embryo for the production of 

 future generations ; other ids disintegrate into determinants, the determinants 

 enter the embryonic cells that result from segmentation, and there themselves 

 disintegrate and set free into the cytoplasm their constituent biophors ; thus 

 they determine the future character of the cells of the organism. The division 

 of primary constituents into those that shall remain latent and those that shall 

 become active is effected largely by the stimulation of external influences ; 

 hence, given several potential formations in the germ, external influences 

 decide which one shall become the actual structure in the adult organism. 

 Once set free and having become somatoplasm, neither the biophors nor the 

 determinants are able to return to the germ-cells. In the adult, germ-plasm is 

 never capable of reflecting in any way the characteristics of the somatoplasm 

 which surrounds it on all sides. With its ancient ancestry it leads a charmed 

 existence, largely independent of environmental changes. It follows that 

 characters acquired by the adult are incapable of acquisition by the germ- 

 plasm, and hence may not be transmitted. The non-inheritance of acquired 

 characters is thus another of the fundamental tenets of Weismann's theory, 

 and one about which he is most positive. 



If these two principles of continuity of stable germ-plasm and non-inheri- 

 tance of acquired characters be true, why are not all individuals in any one 

 line of descent exactly like each other? How is congenital variation possible? 

 In the first place, Weismann allows that germ-plasm, while eminently stable, 

 is not absolutely so ; it is subject to slight continual changes of composition 

 resulting from inequalities in nutrition ; and " these very minute fluctuations, 

 which are imperceptible to us, are the primary cause of the greater deviations 

 iu the determinants which we finally observe in the form of individual varia- 

 tions.' 7 The accumulation of minute deviations may be aided greatly by sex- 

 ual reproduction, or, to use Weismann's more exact term, which is equally 

 applicable to the combination of sexual elements in sexual organisms and to 

 the process of conjugation in the asexual forms, amphimixis. Given the in- 

 finitesimal beginning of a variation, the mingling of two lines of descent, with 

 different past surroundings, may be a most powerful factor in strengthening 

 the deviation and bringing it into recognition as a new character. Moreover, 

 natural selection becomes here also potent as soon as the variation has assumed 

 sufficient proportions to be seized upon by this important factor of evolution. 

 In cases of reversion Weismann supposes the determinants to remain inactive 

 in the germ-plasm for one or more generations and later to develop. The 



