168 
Development of Hook-wornix 
at a much higher temperature (113° F.) if not too prolonged and provided 
decomposition was prevented. Half an hour’s exposure to a temperature 
of 130° F. can be withstood by the fully developed larvae without loss 
of vitality. 
The lower limit of temperature was fixed by Boycott at 59° F. but 
Looss has shown that development can proceed, albeit slowly, at a 
temperature as low as 48° F., and that temperatures only a few degrees 
above freezing point do not kill the eggs provided the exposure is not 
too prolonged; development, however, is arrested during the time of 
exposure below 48° F. but proceeds again when the temperature rises 
above that point. The fully developed larvae are apparently much 
more resistant to cold for, according to Oliver (1910), they will withstand 
burial in snow for at least six days. 
While having no reason to doubt the accuracy of Looss’ observations 
I am inclined to regard them as of somewhat theoretical interest. 
From a practical point of view I must regard Boycott’s opinion as 
representing the facts as they are presented to us naturally, e.g. in the 
matter of the geographical distribution of the worms. Otherwise it is 
difficult to understand why infection did not spread above-ground in 
the Cornish mining district during the summer months, when for six 
months the mean temperature is over 50° F.; nor again is it easy to 
understand why the infection has not spread into the southern parts 
of Australia, where even as far south as Melbourne the mean annual 
temperature is 58° P’. It cannot be that other conditions apart from 
temperature are different, except it be some hitherto unrecognised 
condition, such, for instance, as the wet bulb temperature or the 
humidity of the atmosphere. It seems to me that while Looss’ estimate 
of the critical temperature as 48° F. is probably correct under the most 
favourable circumstances available, yet the disadvantages of a less favour¬ 
able natural environment necessitate a considerably higher temperature. 
Looss is slightly obsessed by the doctrine that what is natural in the 
biological world is best or most favourable, and he is inclined to lose 
sight of the fact that human ingenuity can sometimes improve on 
nature, as in the case of his charcoal method of rearing hook-worm 
larvae. 
Little more than has already been done need be said on the subject 
of the necessity for moisture in the development of hook-worms. Only 
a very small quantity is necessary, while on the other hand a large 
e.xcess is regarded as harmful. It is pointed out by Looss, however, 
that this is not due to the action of the water, as such, but is the result 
