APPENDIX A XLI 
other scourges of humanity. Not till 1883 was it demonstrated by 
Laveran that the blood cells of a malaria patient are being destroyed 
by a little parasite, and not till the end of last century was it shown 
that this is introduced by the bite of a mosquito, in which insect, the 
definitive host, it undergoes some of its important reproductive phases. 
It would be interesting to speculate on the evolution of these blood- 
parasites which require for the accomplishment of their life-cycle two 
host-animals—insect and vertebrate. There is evidence to show that 
many of the allied forms are confined to the former; further, that the 
blood-sucking habit of these is secondary, so that we may attribute 
to this new acquisition the change in habit of the parasites. 
Allied to the malaria parasite are certain forms. (Piroplasma) 
which produce wide-spread diseases (such as the Texas cattle-fever) in 
domestic animals and which are distributed by ticks, while more dis- 
tantly related are the Trypanosomes which produce the Sleeping-sickness 
of Man in Tropical Africa, as well as maladies which ravage domestic 
animals in different parts of the tropics. These are likewise distributed 
by insects, the so-called tsetse-fly and its allies, so that the insect world 
has much to answer for in the spread of disease. How much this 
knowledge has benefitted mankind can be gathered from the successful 
methods adopted to clear parts of Europe of malaria and portions of 
Cuba and Central America of malaria and yellow fever. 
The public has still to realize that the domestic fly, although 
merely a vehicle, is one of the chief agents in the distribution of ty- 
phoid. 
UzrrA-Microscopic ORGANISMS. 
Whether the yellow fever parasite likewise belongs to the animal 
kingdom, as might be suspected from the fact that another kind of 
mosquito is responsible for causing infection is not known, for the 
parasite is so small that it is invisible under the highest powers of the 
microscope and can pass through the finest porcelain filter. 
Ample evidence is forthcoming of the existence of such ultra- 
microscopic organisms, not so far in the free state, but as parasitic 
pathogenic creatures responsible for producing various diseases, such 
as the foot and mouth diséase of cattle, anterior poliomyelitis in Man, 
etc. The discovery of the existence of such invisible organisms puts 
the question of the origin of life in a new position, for instead of being 
due to “some moss-grown fragment of another sphere,” as Lord Kelvin 
suggested, or to the spontaneous generation of visible Monera, life on 
this planet may have been initiated by the evolution of complex but 
infinitely small protoplasm molecules from simpler compounds. 
But the synthesis of living protoplasm, if attainable, is still un- 
attained. Perhaps we are nearer it than in 1874 when Tyndall said in 
Proc. 1911. 4. 
