43 



supplanted by them for the greater part, though not quite. One of these is either com- 

 pletely or almost completely dark, the other grows much more slowly than the normal 

 form and is almost motionless, while the normal form is extremely motile. I will 

 call these variants Ph. indicmn vnt. obscurum and Ph. indicum vnt. parvum. Later I 

 found some more variants which are less common. There are besides sub-variants 

 of which I have examined those standing between the normal form and obscurum; they 

 produce now and then atavists, and vary also towards obscurum, but can be kept con- 

 stant by colony-selection. 



Notwithstandig this great variability it has been possible, likewise by means of 

 colony-selection, during the more than 13 years continued laboratory culture, to keep 

 up the stock unchanged, which is remarkable, when thinking of the place where it 

 was first found. 



The variants and sub-variants always spring from the stock in the same way. 

 They may be reduced to two types: variable and unvariable. All phosphorescent 

 variants are more or less variable. 



The variant parvum shows an extreme disposition for atavism, so that already 

 after its first-re-inoculation on a new culture-medium, various normal forms spring 

 from it. 



The obscurum-rariants are more constant. They are either perfectly constant, 

 so that, as it seems, phosphorescent forms never again arise from them, or imper- 

 fectly, so that after going through a few cell-partitions, answering to as many sub- 

 variants, the normal form returns with the full phosphorescent power. Dark variants, 

 in this way producing luminous cultures, prove that progressive variability 1 ) also oc- 

 curs in the laboratory cultures. 



The variant is not the product of a single heterogene cell-partition, but of the passing 

 through some preparatory cell-partitions, answering to as many sub-variants. I was 

 able without difficulty to distinguish two of these leaps or sub-variants, but it is 

 possible that there are more, too slightly differing for my observation. It is also 

 probable that by the conditions of culture, these preparatory cell-partitions, and with 

 them the sub-variants, existing between the normal form and the main variant, will 

 grow more or less numerous. 



The obscurum-variant is probably produced in accordance with the scheme of 

 Fig. i. 



For the sake of simplicity here is only figured one intermediary stadium (sub- 

 varinat) by the dotted rod; the dark main variant is drawn black, the normal form 

 white. This scheme answers to what may be called the development of the cell-variant 

 by heterogene cell-partition or evolution. 



Less probable is the development of the variant by transformation or epigenesis 

 represented in Fig. 2. 



The preparatory cell-partitions at the atavism of the luminous normal form from 

 the dark or feebly luminous sub-variants still liable to retrogression, probably answer 

 likewise to the schema of Fig. i. 



') Distinction can be made between: retrograding or analytic variability, in which 

 a characteristic disappears entirely or partly, replacing variability, in which a charac- 

 teristic is wholly or partly supplanted by another, and progressive or synthetic varia- 

 bility, in which a new characteristic is added to those already existing. 



