246 THE DISTRIBUTION AND CHARACTERS OF REPTILES. 



phunhcHS, found in the Transvaal low country, will probably be 

 found to merge gradually into A. meleagris. 



In many genera of reptiles, systematists make use of the 

 wide differences in the number of repeated parts for the separa- 

 tion of species : for instance, we might separate the lizard genus 

 Zonunis into its species, merely by counting the transverse rows 

 of body scales. Now such " meristic " variations, though they do 

 not introduce new characters, yet inasmuch as they are definite 

 and constant, might be styled mutations ; but as a matter of fact, 

 they are usually very indefinite, though there mav be disconti- 

 nuity between the speecies. In some species of snakes, e.g., the 

 mamba. the number of longitudinal rows of scales on the body 

 is highly variable. But in the genus Psammophis the various 

 species show much less variability in this respect and amongst the 

 chief characters used by systematists for separating P. crucifer 

 from P. Sibilans is the occurrence of 15 longitudinal rows of body 

 scales tin the former and 17 rows in the latter, yet specimens of 

 crucifer with 17 rows of body scales are known. The same two 

 species may be distinguished by the number of subcaudal scales, 

 but. as the observed range is from 62 to 8t in crucifer, and from 

 86 to 104 in sibilans, we cannot describe this form of variation as 

 other than fluctuational. and no doubt with more material the 

 two series will (l>e found to overlap. Chlor aphis uatalensis again 

 has 114 to 140 subcaudal scales, whereas C. hoploc/aster has 85 

 to 107. and the Rhodesian species C. neglectus hns yy to 114. 



An apparently good instance of a mutation character is 

 found in the si)ecific characters which separate the smallest South 

 African gecko. Lygodactylus capcnsis, from its ally L. ocellatus. 

 The former has the margin of its mental scute deeply cut on both 

 sides, but in the latter the margin of the mental scute is entire : 

 these conditions appear to be quite constant, and I have found no 

 intermediates. It is of interest to note that two Madagascar 

 species of Lygodactylus, which are distinct from ours, show pre- 

 cisely similar differences, probably indicating a great antiquity 

 for this " mutation." 



Again, though there is abundant evidence of much inter- 

 grading of characters within the species of a genus, yet, as Gadow 

 has pointed out, such intergrading is not necessarily the same 

 thing as continuity. Our 17-scaled specimen of Psammophis 

 crucifer is in every other respect typical of the species, and is not 

 to be regarded as an intermediate between sibilans and crucifer. 

 Dr. Duerden. in his admirable work on the geometric tortoises, 

 found very much intergrading in respect of nearly all the charac- 

 ters he employed for the discrimination of the species, and 

 expressed the opinion that they furnished no evidence in favour of 

 the mutation hypothesis : but though he found three distinct types 

 of combinations which were sufficiently separated from any tran- 

 sitional forms to be given specific rank, yet he was so impressed 



