Pkbsidknt's Address — 8kct. D. 147 



strengthened by tlie discoveiy of the bacteria poisons, the toxins, and 

 ol' tlie anti-poisons, tlie anti-toxins. Only the di])htheria bacillus is 

 capable of secreting the diphtheria toxin, and only this toxin pro- 

 duces in the body of animals the speciHc anti-toxin. Thei'e is no way 

 of proving the existence of the toxin except by its specific affinity 

 to its anti-toxin. Thus a number of micro-organisms, in wliich mor- 

 pliological differences have hitherto not been detected, can at pre- 

 sent only be identified by the specific activity of their poisons. 

 Yet at the back of this there must be a definite constitution of 

 the protoplasm of each species. In spite of innumerable attempts, 

 nobody has liitherto succeeded in turning the hay bacillus into the 

 anthrax bacillus, although the virulence of the latter can be modified 

 within very wide limits. Judging from the clinical course of the 

 bubonic plague, there can be no doubt that the bubonic plague 

 bacillus lias lasted unchanged for hundreds of years. At the same 

 time it is well known that many pai-asitic micro-organisms, like the 

 tetanus bacillus, can live as saprophytes outside the bodies of their 

 usual hosts without losing their infectious character, and this shows 

 again the wide physiological range of some species. Amongst other 

 simple organisms in which an enormous specific per.sistenc3^ has been 

 observed, I would point to certain diatoms which have remained un- 

 changed, as far as their skeletons can show us, since the carboniferous 

 period. 



As regards higher animals and plants, numerous instances of high 

 constancy of species are known. At an early stage of the controversy 

 following on Darwin's publications, Schweinfurth pointed out the 

 identity of some of the plants found in ancient Egyptian tombs 

 with plants living at the present day, and Carruthers pointed out 

 the difficulty of reconciling this fact with Darwin's theory ; but we 

 <jan find similar difficulties wherever we turn. I may mention that 

 in my work on South African succulents I constantly refer to figures 

 and descriptions published a couple of hundred years ago, and I fre- 

 x^uently find that our species can be accurately identitied by them. 

 But we have proofs that many of the highly developed species have 

 persisted unaltered very much longer, again during whole geological 

 periods. Will anybod}^ e.g., deny that some of the plants connnon to 

 the neighbourhood of Grahamstown and Capetown nnist have parted 

 company thousands of years ago ? 



But there are more striking examples. Take, for instance, the 

 species of plants occurring at the south corner of America, which are 

 identical with those of the Arctic or northern temperate zone. What- 

 ever the cause of this curious distribution is, these species show us a 

 constancy of type that has to be emphasised when we see people 

 looking down with contempt on " species-mongers." 



I will give two more illustrations of the persistency of specific 

 types through geological periods. The first refers to fishes on both 



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