xvi SYNOPSIS 



pigs given unwholesome food — faeces again examined — B. proteus found in 

 four cases. Conclusions — suggest presence of secondary invaders in experi- 

 ments quoted— possible error from identifying organism by agglutination — 

 variable agglutination of paratyphoid organisms. Application to experiments 

 quoted. Results no evidence of transmutation. (116 — 139) 



CHAPTER X 



SUMMARY 



All characters of bacteria show variation — "spontaneous" or "impressed" 

 — modifying influences already discussed (Chap. ii). Variation may be ap- 

 parent only. Apparently spontaneous variation may be due to unrecognised 

 influences. Variation itself may be specific (morphology of B. diphtheriae 

 and S. scarlatinae). No one character specific — ^variation need not imply 

 loss of specific character (morphology of B. eoli). Analogy of regiment of 

 soldiers and crowd of pitmen. Variation may be specific because it indicates 

 racial character. Many variations represent past stages in evolution (Mor- 

 phology, Chap, iv) — others represent new steps in evolution (Fermenting 

 Power and Virulence, Chaps, v and vi). 



Transmutation differs from variation in degree only— difierent 

 species derived from a common stock — differentiation more advanced in 

 some than others — reversion therefore differently interpreted — necessity of 

 regarding characters as a whole and their stability — danger of relying upon 

 one character alone already shown (Chap, rv-vii). Analogy of human race 

 groups. Variation may indicate recent environment — and so reveal source 

 of particxdar strain (streptococcus from milk — general coli infection from 

 biliaiT^ passages). 



.Stability of variations. "Spontaneous" variations. (1) Imperfect 

 development — tend to disappear. (2) Senility or lowered vitality — tend to 

 persist. (3) Atavistic tendencies — tend to recur. (4) Fresh stage in evolu- 

 tion — ^therefore unstable. Two variations constantly associated — ^both due 

 to lowered vitality — both due to higher evolution — both due to imperfect 

 development or degeneracy. "■Impressed" variations — may lapse when in- 

 fluence withdrawn — may persist for a time — ^may appear permanent — danger 

 of assuming variation is permanent — examples — ability to ferment sugar or 

 produce pigment — inability to ferment sugar or display virulence. Duration 

 of impressed variation. (1) If only part of strain varies it may appear to 

 revert — danger of assuming reversion has occurred. (2) If readily acquired 

 is long retained — if slowly acquired quickly lost (ability of B. typhosus to 

 ferment) — not true of spontaneous variations. (3) The longer the training 

 the more lasting the effect (streptococcus at different stages in disease — 

 ability of B. typhosus to ferment). Same principle governs development of 

 races. Absence of reversion does not imply inability to revert (pigment 



