504 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 one another? 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 

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

 tions." 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 

 theory accounts for the regeneration of lost parts by the assumption that the 

 cells in the vicinity of the wound, by the proliferation of which the new part 

 grows, contain, besides the active determinants that have given them their 

 specific character, other determinants that are latent until the opportunity for 

 regeneration arrives. Some cells do not possess such latent determinants, and 

 hence some parts of a body are incapable of reproducing lost parts. 



Such are the main features of Weismann's theory — a germ-plasm of highly 

 complex architecture and independent of somatoplasm ; continuity of germ- 

 plasm and non-inheritance of acquired somatic characters tending to preserve 

 the uniformity of the species; slight nutritional variation of germ-plasm and 

 sexual reproduction tending to destroy that uniformity; the result is inherited 

 re-eiublance and congenital variation. The theory is now being most actively 

 discussed. 



Theory of Epigenesis. — Among epigenesists no one theory may be said to 

 be pre-eminent. The main features of the epigenetic conception, already 

 referred to, may be summarized as follows: The fertilized ovum is isotropous, 

 i.e. all parts are essentially alike; germ-plasm probably consists of minute 

 particles, but these particles do not represent definite cells or groups of cells 

 ot' the adult ; segmentation is a quantitative process; the early blastomeres 

 are essentially alike, and any one of them, if isolated from the rest, may 

 give rise to a whole organism, although under ordinary circumstances they 

 react upon each other in bringing about the resultant individual ; there is 



