PHYSIOLOGICAL VARIATIONS 41 



growth had taken place, had stood the greatest amount of heat ^ When 

 this process has been continued for some months it is possible to obtain 

 a culture, the individuals of which are quite similar in appearance to 

 those in the original culture, but different from them physiologically in 

 being able to stand a greater amount of heat without having their 

 vitality impaired. The instances of physiological, unattended by mor- 

 phological changes, could be indefinitely multiplied, but want of space 

 precludes a fuller discussion of the subject in this book. 



From the above remarks it will be seen that great care must be 

 taken in the diagnosis of species, and we must not necessarily refuse 

 to believe in the identity of two species merely because they differ in 

 one or two characteristics. The method followed by bacteriologists 

 is to study the life-history of a species in all its aspects, and examine 

 it under different conditions before passing judgment as to its position 

 or name. Only in this way, by judging of the sum-total of charac- 

 teristics, is it possible to eliminate mistakes arising from physiological 

 variations. 



§3. RELATIONS OF BACTERIA AMONG THEMSELVES. 



The complicated relations of life are nowhere more manifest than 

 among bacteria. It must be borne in mind that bacteria are constantly 

 eating up food-material, and what is more important, constantly 

 excreting their waste matters, which are usually poisonous to them- 

 selves and to other bacteria. Then their power of reproduction, when 

 conditions are favourable, is so rapid that if, in any particular nutrient 

 medium, one species is slightly more favoured than the others, this 

 produces such a large number of individuals that the others are either 

 swamped out of existence or are present in very small proportionate 

 numbers. If, however, the conditions change ever so little, and the 

 change is more favourable to one of the weaker organisms, this in its 

 turn multiplies so rapidly that often after a comparatively short time 

 it appears to be the only organism present. This, again, may be 

 replaced in predominance by a third form. That changes are con- 

 tinually taking place is evident, when we reflect on the injurious 

 excretion products that are being continually formed ; some of these 



iWhen growth has taken place after the culture has been subjected to heat, it 

 must not be imagined that all the individuals in the culture have resisted the 

 heat. What has happened is that all except a few have been destroyed, but that 

 the multiplication of these few has made good the loss. 



