CH. X] SUMMARY 145 



in young or backward cultures. We have spoken of the atypical 

 morphology of a young culture of the Klebs-Loeffler bacillus, 

 which renders it difficult to distinguish it from Hofmann's 

 bacillus (vide p. 42), and of its inability to ferment glycerin and 

 lactose (vide p. 55). Such diiferences are comparable to the 

 juTenile features and unskilled hands of a class of schoolboys 

 and tend to disappear of their own accord as the strain grows 

 and develops. 



2. Other variations represent senile changes or are due to 

 lowered vitality, and are seen in old or worn out strains. The 

 loss of motility, or of pigment production, in an old culture 

 will serve as an example. Such variations are comparable to 

 the slow steps and grey hairs that characterise a party of old 

 men and will tend to become more and more developed unless 

 some external influence intervenes and, by effecting a radical 

 change in the conditions of growth, contrives to rejuvenate 

 the strain. 



3. Others again are degenerative in character or are due 

 to atavistic tendencies — such as, for example, the appearance 

 of branched and clubbed forms of the tubercle bacillus. These 

 variations are Comparable to some forms of mental impairment 

 in a family, or to defects such as harelip. They may be passed 

 on from father to son and so persist, or they may disappear, 

 but in the latter case they tend to recur in a later generation. 



4. Others, finally, are evolutionary in character and repre- 

 sent a higher specialisation on the part of the organism — such 

 as, for instance, the development by B. typhosus, after a long 

 training, of power to ferment lactose, or the acquisition on 

 the part of a feebly pathogenic organism of the quality of 

 extreme virulence. Such changes are analogous to the de- 

 velopment of a national genius for literature or conquest. 

 The more highly specialised a function is the more easily does 

 it become deranged and a character, therefore, of this kind, is 

 readily lost. For example, however permanent other newly 

 acquired characters in bacteria may appear to be, variation 

 in the direction of increased virulence seldom is so and alniost 

 invariably proves unstable. 



It is easy to see how, in every one of the four classes we 

 D. 10 



