THE PRESIDENT'S ADDRESS. 211 



dal by a narrow arm of the sea, a mile or so wide, I was assured by 

 the natives that the mosquito is comparatively scarce. What a 

 very small proportion of the Tromsdal mosquitoes can ever have 

 sucked blood, and yet it is suggested that they are parasites. 

 Again, take a leech, although here the question is more diffi- 

 cult. It is admitted on all hands that a lion is not a parasite, it 

 kills its prey and eats it ; but it is said that a leech is, because it 

 docs not kill its prey; but if it happen to attack a very small 

 creature it probably does kill it, and surely the question of whether 

 it is a parasite cannot depend on the size of the particular creature 

 attacked. On the other hand, it seems very odd to be told that the 

 Mallophaga and Dermaleichi, which live all their lives, in every 

 stage, upon particular birds, and will not live elsewhere, are not- 

 parasitic. Of course it clearly is not necessary in order to con- 

 stitute parasitism that the residence of the parasite upon the host 

 should be permanent ; it may be temporary only ; but it does seem 

 to me that it must be more than seizing the so-called host in order 

 to make a single meal. Confining ourselves for the moment to 

 animal parasites upon animals, may we not say that a parasite is 

 a creature which, at the time spoken of, is residing in a permanent 

 or temporary manner in or upon another living creature, and is 

 existing at the expense of or by the assistance of the host. Of 

 course no general definition is likely to be exhaustive, or quite to 

 meet every case ; for instance, the well-known British parasitic 

 anemones, Sagartia parasitica and Adamsia paliata, are found, the 

 former on the back and the latter round the mouth of univalve shells 

 which have lost their molluscous tenants and have been taken pos- 

 session of by soldier-crabs ; it may be said that the anemone does 

 not live upon the crab, but only on the roof of his house. There is, 

 however, a singular instance of one of the foreign crabs which usually 

 has an anemone adhering to each large claw, and goes about waving 

 them in a manner which doubtless renders the Actinia's fishing 

 operations very easy. 



Another difficulty, however, occurs far more serious than such 

 small points as that last named ; there are vegetable parasites as 

 well as animal. Milne Murray, in the " Encyclopaedia Britannica," 

 says " the name of parasites has been given to those plants which 

 are nourished wholly or partially at the expense of other living- 

 organisms," and this seems fairly to express what is the ordinary 

 idea, both scientific and popular. Not only are such plants as the 



