1911.] NATURAL SCIENCES OF PHILADELPHIA. 163 



hill-tops. In a given time the valley and gully forms can attain a 

 much larger size than the hill-top forms. 



There is one dry season in the island and, in general, in the West 

 Indies, during the months from December until the " spring rains" 

 begin, in April or May, and during this dry time the showers are rare. 

 For the rest of the year they may occur at any time, with maximum 

 rainfall in May and November in most years. During the dry season, 

 all of the Pleurodontes are more or less inactive ; when the rains begin 

 in April, they at once begin to pair. The young born one summer 

 have grown to the adult state and pair the next summer. Those 

 which have had the greater number of days of activity or of growth — 

 the gully and lower-ground forms — will hence reach a larger size than 

 those which have had, say, half as many growing days during the wet 

 season; all have been inactive during the dry season. In the two 

 cases the growing days may stand as one to two, the inactive period 

 of the dry season is common to both, and after the same lapse of time 

 in each case the animal becomes adult. Naturally the form that 

 could grow on the greater number of days and the form that had the 

 most abundant food supply will attain the larger size. The diminu- 

 tion in size on the hill-tops can thus be connected with the conditions 

 of moisture, the supply during the growing season being less in these 

 situations. 



The rise of the spire is effected by a closer coiling of the whorl or 

 the suture line drops more rapidly as the spire rises, and the diameter 

 of the coil, the cross section of the aperture, does not increase so rapidly 

 as when the spire is flatter. In a subsequent paper on the growth of 

 the shell I hope to treat this matter more at length. Any mechanical 

 injury to the shell results in the dropping of the whorl below the 

 periphery and a consequent rise of the spire. Lessened vitality, due 

 to insufficient food supply or mal-nutrition of any kind, or in general 

 what may be considered pathological conditions, seems to produce 

 the same effect — a dropping of the coil and a consequent rise of the 

 spire. Frequent stoppages of the growth of the animal due to periodic 

 times of dryness would probably have the same tendency. On the 

 hill-tops we find these conditions of periodic dryness and mal-nutrition 

 during the season when the forms living in the lower ground are grow- 

 ing continuously under optimum conditions of moisture and food 

 supply, and these conditions seem to me to be the controlling factors 

 in the variation in size and form of shell observed, aside, of course, 

 from the effects of isolation on hybridity. 



Isolation under such conditions as obtain upon the hill-tops results 

 eventually in an adaptation of the substance of the organism to its 



