INTERNAL CAUSES OF VARIATION 173 



Thus, while the plan is different in plants and in animals, the 

 first stage, sporogenesis, in plants seems fully comparable with 

 maturation in animals, and the same general end is accomplished. 1 



After all, the manner of division is primarily of interest to 

 the physiologist and does not concern us. Our interest is in 

 the fact that maturation in general involves an actual loss of 

 chroinatin matter (hereditary substance) and a reduction in the 

 number of chromosomes ; and consequently of physiological units. 

 In the present state of knowledge it seems safe to assume that 

 both these results follow, whatever the mechanism of maturation 

 in each particular instance. If this be true, here is a fertile and 

 initial cause of profound variation, an excellent opportunity for 

 losing important elements of the physical make-up, but, so far 

 as we can see, no chance for positive gain, unless it be by new 

 combinations, because nothing is introduced. 



Phenomena such as these are remarkable for what they sug- 

 gest rather than for conclusions that can be positively drawn. 

 The suggestion is that of substantial deviation in the very fun- 

 damental process of transmission of the hereditary substance, 

 a deviation that cannot but be fruitful of variation in resulting 

 individuals. 



Weismann's prediction. It is noteworthy that reduction was 

 predicted by Weismann on purely theoretical grounds some 

 years before it was known as a fact. 2 He argued for its recur- 

 rence as a physiological necessity to prevent the piling up of 

 " ancestral idioplasm," the physiological units to which he 

 afterward gave the name of " ancestral units," 3 and later 

 developed the intricate system of biophors, 4 determinants, 5 ids, 

 and idants, 6 by virtue of which he explained the constitution of 

 the germ plasm, 7 and which he used as the basis for his famous 

 theories of heredity. 8 



1 For full discussion, see articles by B. M. Davis, American Naturalist, XXXIX, 

 Nos. 460 and 463. 



2 Weismann, Essays on Heredity, I, 357, 363-396; II, 114-150; also Weis- 

 mann, The Germ Plasm, chap. viii. 3 Weismann, Essays on Heredity, II, 116. 



4 Weismann, The Germ Plasm, pp. 40-53. 5 Ibid. pp. 53-60. 



6 Weismann, Essays on Heredity, II, 136-138; Weismann, The Germ Plasm, 

 PP-_6o-75. 



7 Weismann, The Germ Plasm, chap, i, pp. 37-85. 

 s Ibid. chap, ix, pp. 253-293. 



