128 A. II. Clark — Reoent Or inoid Faunas. 



Every marine fauna is composed of more or less permanently 

 fixed animals (the most strictly sessile being the crinoids), purely 

 pelagic animals, and animals representing all the intermediate 

 stages. Each class probably has a special fauna! cycle of its 

 own independent of all the others, and independent of the sys- 

 tematic affinities of each of the species included in it. 



Therefore, to apply the principles governing the develop- 

 ment of faunas to a given fauna, we must first of all be famil- 

 iar with the life history of each of the component types, and 

 of the scope of its tolerance to changed conditions, most 

 weight being given to the facts indicated by the most strictly 

 sessile animals and those most intolerant of any change in their 

 environment (among recent forms the crinoids), and the least 

 to those pelagic species which appear capable of existing any- 

 where. , 



Faunas, like individuals, species and genera, pass through a 

 period of youth, of adolescence, of maturity, and of senescence. 



In a very young fauna the various genera are represented by 

 several species each, and each of these species is very variable ; 

 all of the species are near the mean in their respective genera, 

 none being highly specialized and none retrogressive. 



Introduced species which become acclimated and thrive in 

 their new surroundings are found to be, where they have been 

 studied, exceedingly variable. This is equally true in regard 

 to fish, birds, mammals, molluscs and insects, and probably 

 holds good throughout the animal kingdom. We have numer- 

 ous illustrations of this in such animals as have been intro- 

 duced into North America from Europe, Africa and Asia. 



A young fauna is in effect a fauna composed of species all of 

 which are recently introduced, and all of which, maintaining 

 themselves under optimum conditions, with a minimum of 

 parasites and a maximum of food, are able greatly to increase 

 their coefficient of variation. 



Such a fauna we appear to have in the Bering Sea. The 

 crinoids of the shallower waters here are abundant, but all the 

 species, which are very variable, belong to the genus Solano- 

 metra, an intrusion from the antarctic regions. Of the other 

 echinoderms the starfish present a wealth of forms maddening 

 to the systematist ; the number of varieties and incipient and 

 valid species produced from the Ctenodiscus, llippasteria, 

 Solaster, Uenricia, and other types, is almost incredible. 

 Conditions are the same among the echinoids, and among the 

 ophiuroids, and apparently among many, if not most, other 

 animal groups as well. Yet with all this variability there is 

 but a slight tendency to produce pathological, defective, or 

 unbalanced types, types which depart widely from the genera 

 mean as calculated from a study of the same genera in other 

 areas. 



