16 ECHtNODERMA OF THE INDIAN MUSETTM , PART VII. 



It is possible, through a detailed study of the component species of the 

 various subregions which collectively make up the East Indian faunal region, to 

 arrive at definite conclusions in regard to their comparative age ; for faunas, like 

 individuals, species and genera, pass through a period of youtli, of adolescence, of 

 maturity, and of senescence. 



In a very young fauna the various genera are i-epresented 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 retrogres- 

 sive. 



Introduced species which become acclimated and thrive in their new surround- 

 ings 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 prob- 

 ably holds good throughout the animal kingdom. We have numerous illustra- 

 tions of this in such animals as have been introduced 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 predaceous enemies and a maximum of food, are 

 able greatly to increase their coefficient of variation. 



Adolescent faunas exhibit a comparative stability of specific types, coupled 

 with the incipient formation of new genera as a result of a growing tendenc}^ of 

 the species to depart widely from the generic mean. 



In mature faunas the species are fixed, save only for the species at the mean 

 of each genus, which always remains variable, and new generic types are found 

 which have become separated off from the parent genera through the suppression 

 of intermediates, or have arisen by discontinuous variation. As a result of the 

 formation of these new generic types the number of species in each genus is 

 diminished, and the species are found to approach more or less closely the means 

 of the original genera, or the means of the genera newly formed. 



Senescent faunas have lost a considerable proportion of the genera which 

 they possessed at maturity ; the genera which I'emain include aberrant species in 

 which certain characters have become greatly exaggerated, giving to tliese species 

 a curiously unbalanced appearance. There is typically but a single species in 

 each genus ; but there may be two or more, each with a different set of characters 

 exaggerated, in which case they are usually treated as aberrant monotypic genera. 

 A pathological fauna may resemble a senescent fauna in its general facies ; 

 but in a pathological fauna all the species, besides being aberrant, are excessively 

 variable, which is never the case in a senescent fauna. Pathological faunas 

 occur usually on the limits of faunal areas, or on the boundary between two very 

 different faunal areas, and are composed in the latter case of intrusive species 

 from both the adjacent areas. 



Occasionally faunas are found which combine the characteristics of two or 

 more of the faunas described above; these are rejuvenated faunas, faunas which 



