320 THE GERM -PLASM 



the type of the species, and not that of an ancestor, in case it 

 alone has to control the ontogeny. 



The foregoing considerations seem to me to afford a good 

 explanation of the frequent occurrence of reversion in young 

 species which are not yet defnitelij established. Cultivated 

 varieties of flowers, such as the Heartsease (I'iola tricolor). 

 always produce, among a large number of seedlings, certain 

 plants which bear flowers more or less resembling those of the 

 wild species. Evidently only a portion of the idants have become 

 transformed in this modern form, another and smaller portion 

 having remained unmodified. Since the reducing division sepa- 

 rates the idants into two groups of all possible combinations, 

 germ-cells, both male and female, containing a preponderance 

 of unmodified ids, must also occur ; and if two such germ-cells 

 come together in fertilisation, reversion must result. 



Let us suppose that, of the si.xteen idants in the germ-plasm, 

 ten were modified and six unmodified. Under favourable cir- 

 cumstances a case might then occur in which the majority of 

 idants would remain unmodified, without further accumulation 

 taking place in the course of generations — i.e., at every fertilisa- 

 tion. This may be illustrated by the following table, in which 

 the letters with a dash represent the unmodified, and those 

 without one the modified idants. the line dividing the letters 

 indicating the reducing division : 



Maternal germ-plasm ^ a', b'. c, d', e. f. g', h i. k. 1'. m. n, o, p, q. 

 Paternal " I f. r', s, t', u', v'. w'. x y. z, a. b', c, d. e, g. 



Germ-plasm of offspring I. : — a', b', c, d'. e, f, g', h x f, r', s, 

 t', u', v', w', X. 



Owing to the manner in which the reducing division takes 

 place, the maternal and paternal germ-cells each contain five 

 unmodified idants, so that in the germ-plasm of the offspring 

 ten unmodified and six modified idants are present. If. on the 

 other hand, we suppose that the other two cells produced by the 

 reducing division from the same mother-germ-cell undergo ferti- 

 lisation, the germ-plasm of the offspring II. will have the follow- 

 ing composition : — 



i. k. 1'. m. n. o. p, q. y, z, a, b'. c, d, e. g. 



It therefore onlv contains two unmodified idants. and must 

 consequently give rise to the modified form of the plant. 



