No. 48.— 1897.] on Wallace's theory. 



83 



range may be I cannot discover, but I imagine them to be 

 a purely littoral species, and they all crawl freely out of 

 water. The temperature of the surface water over this vast 

 area seems to vary from about 63° F. in its northern and 

 southern limits to 86° off Ceylon. 



In a common fresh water shell found over a great part of 

 India and all the low-country of Ceylon we find the same 

 mixture of varieties over a large area, and under very 

 different climatic conditions, as in the marine species. So 

 great are the varieties that it has been described as 

 Paludomus acutus, modecella, spiralis, spurcus, lotusus, 

 parvus, palustris, and obesus, which, accordingto Blandford, 

 are all synonyms of Paludomus tanjoriencis, Gmelin. These 

 different varieties can in some localities be collected within 

 the space of a few yards. 



The genus Tanalia is restricted to the hills of the Central 

 Province, and to the forest-clothed country of the south and 

 south-west — less than half the area of the entire Island. Its 

 twenty-four species have been referred by Mr. Blandford to 

 Tanalia aculeata, Gmelin, and he says that " in the Kelani- 

 ganga between Kitulgala and Yatiyantota he has collected 

 T. tennentii, T. picta, and T. undulata, with intermediate 

 varieties, Here we have an example of the same mixture 

 of varieties, only over an extremely limited area in closely 

 allied conditions of climate." 



This form of a " Wallacian wood " is extremely common 

 in Ceylon. One of the most remarkable examples of it is 

 Plotheia decrescens, Walker, a tray of which is exhibited. It 

 is almost impossible to find two of these moths alike. This 

 extraordinary variability is the more remarkable in this case 

 as we have another peculiar species, P. strigfera, which 

 also appears to be very variable. Other instances occur in 

 the frogs ,and the reptiles, and I consider that even in our 

 monkeys (Semnopithecus cephalopterus, Zimmer, and Semno 

 pithecus ursinus, Kelaart) we have merely varieties with a 

 strong tendency to produce a white variety, which is well 



