1918] Essenherg: Distribution of the Polyiwidae 191 



D. FACTORS CONTROLLING DISTRIBUTION 



On the basis of the above examples the Polynoidae of this coast 

 may be grouped according to their horizontal distribution into sub- 

 boreal, temperate, subtropical, tropical and cosmopolitan species; and 

 according to their vertical distribution they fall into littoral, sub- 

 littoral, and deep water groups. These facts suggest that there must 

 be some reason for such varied distribution, or else the distribution 

 would not occur in such a constant and definite manner. Assuming 

 that all actions are in response to stimuli, the phenomenon of distri- 

 bution may be best understood by considering the factors or stimuli 

 which play a role in distribution. Many of these factors are un- 

 doubtedly so complex that they can be studied only by complicated 

 methods of chemical and physical analyses. The most obvious factors, 

 however, may be enumerated as follows: (1) temperature, (2) cur- 

 rents, (3) winds, (4) chemical composition of the water, (5) food 

 habits and mode of life of the Polynoidae. 



I. Temperature in Relation to Depth and Latitude 



Before we speak of the influence of the temperature on the oceanic 

 fauna and flora, and of the role it plays in distribution, it is essential 

 to have a clear conception of the relative temperatures of the various 

 depths and latitudes. As far as it is known from the hydrographic 

 records of the various investigations of the oceans, the bottom tem- 

 peratures of the depths of the Pacific Ocean are remarkably uniform, 

 ranging from 1?6 to 1?9C (Schott and Schu, 1910; McEwen, 1915). 

 There is a very slight variation in temperature in abyssal waters 

 below 2,000 fathoms. The bottom waters are the coldest near the 

 California coast. The temperature becomes gradually warmer with 

 the increasing distance from the shore, but the variation of the tem- 

 perature amounts only to a difference of 0?45 Fahrenheit between the 

 longitudes 120° and 160° (Clark, 1916) or from the coast of Califor- 

 nia toward the mid-Pacific. This difference in temperature is so 

 insignificant that it is almost negligible. 



This, however, is not true of the surface temperature, which is far 

 from being uniform ; in fact, considerable fiuctuations in temperature 

 due to various physical causes are found in comparatively small areas. 

 Considering the surface temperature of the entire Pacific coast in 

 general, one finds that it follows certain laws varying according to 



