360 THE BEGINNINGS OF LIFE. 



Quite independently of all this, however, it will be 

 found, as we have already hinted, that an exclusive 

 doctrine of contagion is singularly unsupported, and 

 inadequate to account for the spread and' perpetuation 

 even of undoubtedly parasitic diseases. In this respect, 

 the history of these diseases is essentially similar to the 

 history of those non-parasitic but contagious febrile 

 diseases which frequently prove so disastrous to the 

 human race. A comparison of the respective pheno- 

 mena which characterize them, tends to show that the 

 parasitic and non-parasitic affections are more inti- 

 mately allied than might be supposed. Both sets of 

 diseases are due to the occurrence of spreading changes 

 of a general character throughout the body, leading to 

 grave alterations in the constitution of its principal 

 fluids 1 . These changes in the ordinary exanthemata 

 are not such as to lead to a birth of independent 

 organisms within the blood and tissues 2 ; whilst in 

 other affections they do entail the birth of a multitudi- 

 nous progeny of independent units. And partly because 

 the changes which lead to this birth are probably more 

 complete, and more antagonistic to the ordinary vital 

 processes though even more on account of the enor- 

 mous powers of reproduction possessed by such new- 

 born independent units the higher organisms which 

 are thus affected are nearly sure to die. This almost 



1 See Appendix E, pp. cxvii and cxlvii. 



2 See Trans, of Patholog. Soc.' 1869, P- 



