214 Universitij of California Publications in Zoology [Vol. 18 



factor and a result of distribution. As a factor, because species with 

 greater plasticity and greater power of adaptation are able to live 

 in a great variety of environments, and consequently have more 

 chance to survive than have species with a very limited power of 

 adaptation. Some phases of variation serve directly as a protection 

 to the animal. Other forms of variation are usually results of environ- 

 ment. It is the expression of a varied mode of life, or a response to 

 external stimuli. This has been variously proved by experimental 

 methods, where variations are produced artificially by changing the 

 environment. Not all living organisms, however, respond to the 

 same external stimuli in the same way. While one form of animal 

 life may undergo considerable changes under the influence of external 

 stimuli, others, being unable to adapt themselves to new conditions, 

 will perish. This is shown in Loeb's experiments with the eggs and 

 the embryos of Fundulus referred to above. While a great percentage 

 of the embryos were killed after four to seven hours exposure to low 

 temperature, others remained alive, and among the latter 30 per cent 

 were abnormal. This indicates that there is a difference in plasticity 

 or in the power of adaptation even among different individuals in the 

 same species. In sudden changes of environment the power of adapta- 

 tion will determine the survival and, consequently, the distribution of 

 a species or an individual. 



Numerous instances of great adaptation are known among various 

 organisms signifying that the animals have an innate power which 

 enables them to resist adverse conditions. This power is designated 

 as plasticity or adaptability. The degree of plasticity varies in differ- 

 ent species and individuals. Hence as the distribution depends to 

 some extent on the power of the adaptation of the animals to the 

 changing environment, the plasticity may be justly considered as a 

 factor determining the distribution. 



From the observations made, one comes to the conclusion that the 

 plasticity in Polynoidae, although varying in degree in the different 

 species, is fairly great in most. Marked differences between different 

 genera and species are found among polynoids characterizing their 

 mode of life and the influence of the environment upon them. These 

 differences occur in the size, the shape and the color of the body, in 

 the size, shape, texture and color of the elytra, in the number, shape 

 and size of the setae, in the texture of the cuticle, and in other char- 

 acteristics. These variations occur not only in different species but 

 individuals of the same species which are living in different environ- 



