230 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



to say impossible, that this theory correctly explains the 

 phenomenon, because, since the toxins of every disease 

 differ from those of every other disease, as is proved by 

 the difference of their effects on the organism, this woidd 

 mean that the host is capable of elaborating separate 

 anti-toxins, highly complex chemical substances, exactly 

 antagonistic to the different toxins of all the separate 

 ■zymotic diseases to which he is susceptible, and against 

 which he is able to acquire immunity ; a thing wholly 

 incredible, esjDCcially when we remember that these 

 anti-toxins may, if the theory is true, be manufactured 

 by individuals whose ancestry can have had no experi- 

 ence of the disease, and in whose race therefore there 

 can have been no evolution of the power of such manu- 

 facture ; for instance, the horse is resistant to dijDhtheria, 

 and is able to acquire immunity against it, yet we have 

 no reason to sujspose that the equine race has in past 

 ages been afflicted by the disease ; again, the natives of 

 Australia and the islands of the Mid Pacific are able, 

 though less able than the inhabitants of the Old World, 

 to acquire immunity against such Old World diseases as 

 measles, scarlatina, small-pox, &c., yet, as we know, these 

 diseases are new to their races. Moreover this theory 

 of immunity by the production of anti-toxins is unneces- 

 sary, since the theory of immunity by fit variations suffi- 

 ciently explains the phenomena. We can well believe 

 that in the horse and in the Samoan the amoeboid 

 leucocytes have retained the phagocytic functions of 

 their remote amoeboid unicellular ancestors, and that in 

 these cells there has been an evolution of the power of 

 varying in a fit direction in response to stimulation, 

 whereby they are able to react to stimulation from the 

 toxins, even though these toxins are strange to them 

 and to their ancestry, just as other cells of the body are 

 able to vary in a fit direction when stimulated, even 

 when the stimulation is strange, this being the especial 



