iQoS.] Records of the Indian Museum. 35 



otherwise than that this variable and many-toothed species has 

 been derived from a form which had a smaller number of teeth. 

 This number was probably six, for this is most common throughout 

 the genus. 



One who holds strictly to the idea that such changes are brought 

 about by the perpetuation and accumulation of minute variations 

 which occur in any direction, must suppose that the widely dis- 

 tributed members of the genus are kept constant as regards number 

 of teeth, because that number suits some pe2uliarity of their environ- 

 ment. Any individuals showing variations from that number are 

 usually unable to reach maturity, for abnormal specimens appear 

 among a collection of adults to the extent of only 2 per cent. 



In order to explain how a form having fifteen teeth was derived 

 from one having but six, one must believe, if the theory of gradual 

 change is true, that a series of ancestors having 7, 8, 9.... 15 

 teeth must have existed. Starting from the point when the six- 

 toothed form gave rise to one having seven teeth, one must imagine 

 a change in the environment which favoured the few seven-toothed 

 variations, so that few as they were among the myriads of young, 

 a comparatively large percentage of them began to reach maturity 

 and to transmit the new character to their offspring, until seven- 

 toothed adults formed a majority and a new species was formed. 



By a like process this gave rise to a species having eight teeth. 

 The change of environment which caused the production of the 

 seven- toothed form out of the six- toothed, could not produce the 

 eight-toothed form from the seven-toothed. To produce this a 

 further change in the environment appears to be necessary. This 

 change must have been similar in nature to the first change, for it 

 produced a like effect, but it must have been of increased intensity. 

 If it were not so, the race would remain seven- toothed. In other 

 words there must have been a continuous and increasing change 

 of some particular feature of the environment to have caused the 

 change in number of teeth from six to fifteen. What this change 

 could have been is not easy to imagine. The supposition might be 

 made of a gradual diminution in the size of the favourite prey, 

 favouring an increase in number of the raptatorial teeth. How- 

 ever, if one examines, side by side, the claw of a Squill a with six 

 teeth, and that of one with fifteen teeth, it seems impossible to 

 imagine a small animal which could escape from either when the 

 dact^de is closed down into its opposing groove ; still less can this 

 supposition account for an increase of one tooth at a time. 



The facts of the case, which do not seem to be in favour of the 

 " Theory of gradual change," are recorded here as a contribution 

 to the study of animal variation available for comparison with 

 similar cases which may be recorded. ^ 



