208 PROCEEDINGS OF THE ACADEMY OF [Mch., 



powerful argument for the agency of external irritations as a cause of 

 change in structure, and the further fact that they have become perma- 

 nent in such genera as Stilosoma, Helicops, Haldea, Abastor, Carphophis 

 and others, certainly indicates that these degradational characters have 

 been inherited. They cannot be " ontogenetic" modifications, for they 

 are exhibited by the newly born young ; furthermore, it is of the very 

 nature of ontogenetic modifications that they must be adaptive, 

 while the whole biu-den of the present argument is to show that those 

 here in question are not of that character. That the complete absence 

 of internasals or loreal is sometimes correlated with a shortened snout 

 and reduced dentition is true, but this condition is not adaptive but 

 degradational. The result of adaptation in burrowing species is seen in 

 Heterodon, where the bony snout has been lengthened and made 

 stronger instead of being reduced. In response to external irritations 

 this genus of exceptional burrowers has developed an irregular number 

 of supernumerary plates on the snout, instead of suffering any loss. 



The elongation of the rostral, or turning up of it like a plough, is 

 usually regarded as an adaptation related to subterranean habits, and 

 in some cases, as Heterodon and Lystrophis, it is doubtless true, but the 

 elongation reaches a maximum in Dryophis, which is more arboreal than 

 most snakes, and is wholly absent in many others which burrow freely. 



In body scales the range of individual variation in the number of 

 rows follows a general law of proportion to the normal mean. In 

 ColuhridcB, with a minimum of ten and maximum of thirty-five, they 

 rarely vary more than two to six rows, while in Boidce, with extremes 

 of twenty-one and ninety-five, they frequently vary as much as fifteen. 

 Mr. Bateson's law of "meristic" variation seems to be followed. The 

 power of heredity is shown in this relative adherence to a mean, but 

 with the normal range of variation in mind, it cannot be supposed that 

 a slight difference in the number of rows can ever have had selective 

 value to the ancestral form of any species. The fact that the normal 

 number in a species bears a quite definite relation to girth of body, indi- 

 cates that they are determined by laws of growth. This might be 

 regarded as a correlation, under Mr. Wallace's comprehensive defini- 

 tion, but it may be doubted if size, as a general fact in nature, is con- 

 ditioned by Natural Selection. It is more likely to be determined by 

 laws of growth reacting to nutritive supply. 



The function of keels and scale pits is not known. The last are 

 probably vestigial. Keels may serve a purpose in the swimming habit 

 of aquatic species. In Hydrophince, where they are not much devel- 

 oped on the dorsal scales, ventral keels are common. With both struc- 

 tures probable utility may be conceded . 



The use pertaining to an entire or divided anal plate is not evident. 

 It might appear that a divided anal would be of service in increasing 

 extensibility of the parts involved, but in that case it would be a pro- 

 gressive character, and we have, on the contrary, genera with an entire 

 anal, such as Eutcenia and Pityophis, descended from Tropidonotus and 

 Coluber, which have the divided form. It is the case, however, that 

 an entire anal is present in all but two genera of Boidce, which are near 

 the base of the series in phylogeny. 



