18 THE president's ADDRESS. 



ever, would be a long and gradual one, perhaps attained only by a 

 painful process of natural selection of the hosts. 



On the other hand, if I am right in regarding the members of 

 the 6r?tcn-group as newly-arisen species descended from a common 

 ancestral species of trypanosome, it follows that the ancestor 

 must, through some process of variation, have developed the 

 power of establishing itself in new hosts and invading, so to 

 speak, fresh pastures. It may be supposed, therefore, that if this 

 has happened once it may happen again, and that a parasite 

 which is now specific to a single tolerant host may, by some 

 unexplained process of variation, acquire at any time the power 

 of infecting other hosts and so giving rise to new species of 

 parasites ultimately. The facts of variation are at once the 

 most patent and obvious characteristic of living beings, and the 

 most difficult to trace to their causes ; their study is of the 

 utmost importance for understanding the evolution and the origin 

 of species, and the phenomena of parasitism offer a field for the 

 study of variation of a peculiar kind, that is to say, variation 

 in physiological and constitutional rather than morphological 

 characters. The variations in the powers of parasitic organisms 

 are clearly of the greatest practical as well as theoretical im- 

 portance, when it is seen that the spread of a parasite into new 

 hosts is likely to be accompanied with the most deadly results 

 to the hosts invaded by it. In fact, the origin of species amongst 

 parasites is bound up with the question of the origin of disease. 

 Eay Lankester has suggested* that the extinction of animals 

 seen in past geological periods may have been in many cases due- 

 to their extirpation by some species of parasite new to them. 



Thus it is seen that in the origin of species amongst parasites 

 there are, as in other organisms, two steps : first the appearance 

 of variations, with the resultant disharmony seen in the lethal 

 forms ; secondly, by a gradual process of reciprocal adaptation 

 between host and parasite, the establishment of more normal 

 harmonic relations, associated with definite specific characteristics 

 and reactions on the part of the parasite and host. 



* Quarterly Review, No. 399 (July, 1904), p. 134. 



Jov.vii, Quck-ett Microscopical Club, Ser, 2, Vol. XL, No. OC, April 1910. 



