THE PHYSICAL BASIS OF LIFE 201 



of any pair, say two blacks or two whites, ... or the gametes 

 from which it originates may be bearers of the dissimilar 

 characters, say a black and a white. [In the first case, no matter 

 what its parents or their pedigrees may have been, the zygote 

 breeds true indefinitely, unless some fresh variation occurs.] 



If, however, the zygote be gametrically cross-bred, its gametes 

 [or germ cells] in their formation separate the pair of characters 

 again, so that each gamete contains only one character of each 

 pair. At least one cell division in the process of gametogenesis 

 is therefore a differentiating or segregating division, out of 

 which each gamete comes sensibly pure in respect of the unit 

 characters it carries, exactly as if it had not been formed by 

 a cross-bed zygote at all. 



For our purposes this may be reduced to three propositions : 

 (1) that inheritance consists in the transmission of independent 

 characters, of which each race or species possesses a definite 

 number; (2) that these characters form pairs of opposites or 

 alternatives ; (3) that in the formation of the germ cells these 

 characters are sorted out and distributed so that no germ cell 

 carries both members of a pair. Can any material basis be found 

 for these ? To this we will now turn. 



Five years ago it is doubtful whether there existed in the 

 whole domain of science such a charnel-house of dead facts 

 as in that of the science of cell structure. Thirty years of active 

 study of animal and plant cells prepared for microscopical 

 examination in various ways had resulted in the accumulation 

 of a multitude of details respecting the structure of the cell 

 nucleus and of the extraordinary way in which it behaves in 

 cell division, and especially in those cell divisions which produce 

 the germ cell. It was known that from the characteristic sub- 

 stance of the nucleus — which stains very deeply with aniline 

 dyes, and hence is called chromatin — a continuous thread is 

 spun as the first step in cell division, and that this thread of 

 chromatin splits across into rods called chromosomes, each 

 of which again splits, this time not across, but lengthwise, 

 so as to form two " daughter " chromosomes, which, under the 

 influence of a peculiar field of force formed in the substance 

 of the cell, move away from one another and gather at the 

 opposite poles of the spindle-shaped field, there to fuse and 

 form the two nuclei of the " daughter " cells. 



A further very significant and curious fact was known — 

 namely, that the number of chromosomes formed in the process 

 is not a chance one, but, in the first place, it is always an even 



