Tropisms and Instincts as Adaptations 399 
bacteria have a flagellum at each end, and swim indifferently 
in either direction. If they meet with something that stimu- 
lates them, as they move forward, they swim away in the 
opposite direction, and continue to move in the new direction 
until something causes again a reversal of their movement. 
In this respect their mode of reaction seems of greater 
advantage than that followed by paramoecium. 
Another instinct, that appears to be due to a tropic 
response, is the definite time of day at which some marine 
animals deposit their eggs. The primitive fish, Amphioxus, 
sets free its eggs and sperm only in the late afternoon. A 
jellyfish, Gonionema, also lays its eggs as the light begins 
to grow less in the late afternoon, and in this case it has 
been found that the process can be hastened if the animals 
are placed in the dark some hours before their regular © 
time of laying. There is no evidence that this habit is of 
any advantage to the animal. We may imagine, if we like, 
that the early stages may meet with less risk at night, but 
this is not probable, for it is at this time that countless marine 
organisms come to the surface, and it would seem that the 
chance of the eggs being destroyed would then be much 
greater. It is more probable that the response is of no 
immediate advantage to the animals that exhibit it, although 
in particular cases it may happen to be so. 
This response recalls the diurnal opening and closing of 
certain flowers. The flowers of the night-blooming cereus 
open only in the dusk of evening, and then emit their strong 
fragrance. Other flowers open only in the daytime, and 
some only in bright sunlight. It is sometimes pointed out 
that it is of advantage to some of these flowers to open at a 
certain time, since the particular insects that are best suited 
to fertilize them may then be abroad. This may often be the 
case, but we cannot but suspect that in other cases it may 
be a matter of little importance. In special instances it may 
be that the time of opening of the flowers is of importance 
