COLD LIGHT—-HARVEY 213 
in recent times, we may be dealing with luminous bacteria or there 
may be secreted some easily oxidizable substance that luminesces dur- 
ing its oxidation. Several such bodies are known in organic 
chemistry. 
To the student of evolution, luminous animals offer a great field, 
but a field in which relatively little is known. Almost everyone is 
interested in the use of luminescence to the luminous animals, and 
unfortunately we can say in only a very few cases what the use of 
the light is. Who, for instance, would venture to suggest the use of 
light to a luminous bacterium, an organism which is perhaps one 
twenty-five-thousandth of an inch in diameter and which has not 
the nervous reactions of a higher form; or the use of the light to an 
animal which occurs living at the surface of the sea, and which also 
has no nervous system, a one-cell form, blown hither and thither 
by the wind? 
Apparently, in such cases as this, we must believe that the light 
is merely fortuitous, that it accompanies merely some of the organic 
chemical changes which go on in the animal. It is a chance phe- 
nomenon. On the other hand, it would seem likely that deep sea 
fishes and squid—and it is chiefly these forms which have the lantern, 
complicated in structure—must use their light as a searchlight for 
seeing things in a region where we know light does not penetrate. 
On the other hand, a great many species are known which do not 
live continually in dark places and many luminous forms do not 
move around at all, the sea pens, for instance. They are almost all 
luminous, a colony of animals that live in the mud or sand at the 
‘bottom of the sea at a depth of perhaps 50 feet where there is plenty 
of light. As they do not move about from one place to another it 
has been suggested that they may use the light as a warning. If a 
predacious fish comes along, the minute the sea pen is disturbed by 
the fish, the hight is flashed on. That warns the fish and scares him 
away. But this is a mere conjecture and I think no one has seen it 
take place. It has been thought also that animal light may be used 
as a lure, that certain forms use their lights to attract other forms 
on which they prey. Whether that is true or not, is also a conjecture. 
Finally, it certainly seems that in some forms the light is used to 
attract the opposite sex in mating. That is the case with the firefly. 
Each species of firefly has a light which shines in a certain definite 
way, and if one is an expert, he can go into the field and point out the 
different species of fireflies by the interval between flashes and the 
time of the flashes. The male and female of each species are brought 
together by signaling in that way. 
The chemical nature of animal luminescence is the subject I have 
studied most closely. Whenever I mention that I am interested in 
luminescence, I am always asked one question—whether the light is 
20837—27——-15 
