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SCIENCE 



[N. S. Vol. XXXIX. No. 994 



equalities in cell division and without any- 

 corresponding variations in environmental 

 condition. Or, on the other hand, they 

 may be causally related to changes in the 

 chemical and physical surroundings of the 

 organism as were those which MacDougal 

 produced among the higher plants by in- 

 jecting chemicals into the ovary and such 

 as Tower caused by exposing potato beetles 

 to special conditions of temperature and 

 humidity. Changes of this sort are very 

 familiar among the bacteria, as for exam- 

 ple in the case of the increase in virulence 

 on passage through siisceptible animals, 

 or the converse process of attenuation, 

 as practised in the preparation of vac- 

 cines for anthrax and other diseases. Wolf 

 (1909) reports a considerable munber of 

 temporary modifications and some perman- 

 entty inheritable ones stimulated by expos- 

 ing bacteria to the action of chemicals. 

 White and dark red strains were thus pro- 

 duced from a normal B. prodigiosus, the 

 resulting modifications breeding in each 

 case true to their new type. Variations of 

 this sort called forth by the direct efliect of 

 the environment I have been accustomed to 

 distinguish by the term " impressed varia- 

 tions. ' ' 



The net result of the various sorts of 

 variability to which the bacteria are sub- 

 ject is to produce a condition, not different 

 in kind, but more extreme in degree, than 

 that which exists among more complex 

 forms. As Bateson (1913) says: "The 

 problem, of species in its main features is 

 presented by these organisms in a form 

 identical with that which we know so well 

 among the higher animals and plants." 

 Several peculiar conditions tend, however, 

 to make specific distinctions even more 

 unstable among the bacteria than elsewhere. 

 In the first place the action of the environ- 

 ment upon unicellular organisms is pecu- 

 liarly direct and the fact that all cells are 

 potentially reproductive removes any bar 



against the inheritance of acquired charac- 

 ters. Again the absence of sexual repro- 

 duction must operate to preserve varia- 

 tions which arise from within or without. 

 Among sexual organisms it is true that 

 amphimixis is held to be in itself an im- 

 portant source of germinal variations. Yet 

 this is true only within certain rather defi- 

 nite limits and beyond those limits sexual 

 reproduction ceases or becomes infertile 

 and thus the more divergent variations are 

 eliminated. With fission as the normal 

 method of reproduction, on the other hand, 

 every variation which can arise may be 

 handed on, unchanged. Finally the rapid- 

 ity with which the bacteria multiply fur- 

 nishes exceptional opportunities for the 

 operation of selection or any other modify- 

 ing force. The immense number of genera- 

 tions which may succeed each other in a 

 short space of time might be expected to 

 make boundary lines as shifting as they 

 would become among the higher plants if 

 a dozen geological epoclis were considered 

 all at once. 



There are sharp limits to the variability 

 of even the bacteria however and for prac- 

 tical purposes we find the larger groups 

 quite constant in their general properties. 

 As a rule typhoid germs descend from ty- 

 phoid germs and tubercle bacilli from 

 tubercle bacilli. The same yellow coccus 

 falls on gelatin plates exposed to the air, 

 all over the world. The same spore-form- 

 ing aerobes occur in every soil, the same 

 colon bacilli crowd the intestines of animals 

 and men in every clime. In part at least 

 I am inclined to believe that this is due to 

 the direct or selective effect of similar en- 

 vironmental conditions producing what 

 Jordan and Kellogg call among higher 

 organisms "Ontogenetic species held for a 

 number of generations true to a type sim- 

 ply because the environment, the extrinsic 

 factors in the development of all the indi- 

 viduals in these successive generations, are 



