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 paramcecium. 



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 



