46 INSTRUCTIONS FOR COLLECTING 
On the other hand, it is well known that the flesh of various sea- 
fishes possesses poisonous properties, either constantly or at certain 
seasons or in certain localities. Observations should be made on the 
causes of these properties, on the nature of the poison, and its 
operation on other animals. 
Other fishes, the flesh of which is quite harmless or even well 
flavoured, are armed with special poison-organs. It would be very 
desirable to investigate this poison in fresh fishes, and to experiment 
with it as to its effect on other fishes. 
All observations relative to sex and propagation are of interest ; 
observations on secondary sexual differences, hermaphroditism 
(whether normal or abnormal), the numerical relation of the sexes 
(whether monogamous or polygamous), spawning, and migrations ; 
nest-building, care for the young, etc. The ova of pelagic fishes, 
many with peculiar appendages on the outer integument, are 
frequently met with floating on the open sea, sometimes singly and 
sometimes in clusters. Every opportunity of observing their develop- 
ment, and of tracing the stages of growth of the young fishes, should 
be taken advantage of. 
As long as a species of fish is allowed to exist in a district without 
molestation, it attains to a much greater size than subsequently, 
when it is diligently sought after, and when none of the individuals 
are allowed to reach their full growth. On this ground alone it is 
desirable to collect the largest individuals; but if the traveller is 
unable to preserve them, he should at least record their length and 
weight. 
In hot countries, where the streams and ponds are liable to dry 
up, many fishes are compelled to temporarily breathe air, or travel 
considerable distances overland, or fall into a state of torpidity. 
Observations on such habits, which are verified by the traveller 
himself, are of incomparably greater value than what he learns by 
hearsay from the natives. It would also be interesting to know 
whether the fishes of the Arctic and Antarctic zones are able to 
maintain life when enclosed in ice, and, if so, for how long. 
Many pelagic fishes are provided with smooth white or pearly 
organs imbedded in the skin, which are small and round on the body 
of the fish, but larger and of various shapes on the head, especially 
