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SCIENCE 



[N. S. Vol. XXVIII. No. 714 



dividual snake, are brought about always by 

 the dropping of certain definite rows, and are 

 not promiscuous, and this observation leads on 

 to the almost equally certain conclusion that 

 specific variation in the scale rows follows the 

 same sequence as in the case of individuals, 

 and is correlated with girth of body. Outside 

 of such sequence variation so rarely occurs 

 that it is negligible. So also with the labial 

 scuta, certain of which are present, absent or 

 fused, in dependence upon head-length. 



These results are of much interest and 

 value, and will become more so with increas- 

 ing knowledge of the processes which make up 

 the so-called "laws of growth." But — and 

 here criticism must take a hand — ^however 

 surely the chapter on variation establishes 

 these and other novel facts, the reviewer is not 

 able to see that the proposition laid dovm by 

 Dr. Euthven as a guide to his phylogenetic 

 lines of parallel development necessarily fol- 

 lovre from them. The proposition in brief 

 is that Thamnophis started out with the maxi- 

 mum number of dorsal rows known in the 

 genus, and that the forms resulting from 

 geographical extension are for the most part 

 consequent on dwarfing, due to unfavorable 

 environment — the whole course of species 

 formation in the group being one of reduction, 

 and the maximum of size being assumed to 

 be T. megalops, of the Mexican plateau, with 

 an occasional twenty-three rows. This is a 

 necessary step to the author's final conclusion 

 as to the original home of Thamnophis, but it 

 is by no means certain that megalops in the 

 average is really larger than sirialis or parie- 

 talis of the north, and examples of the ques- 

 tionable form known as hiscutata, from 

 Oregon, are now and then found which also 

 have twenty-three rows. 



Dwarfing has undoubtedly been a factor in 

 the formation of some species, as notably 

 iutleri and leptocephalus, but the evidence is 

 not complete that it has been general. Indeed 

 the fundamental postulate of the theory has 

 more strain put upon it than it can bear, for 

 in the light of what is known as to the rela- 

 tive abundance of garter snakes in different 

 portions of their range, and of their habits, 

 it is not easy to admit that all conditions en- 



countered by them beyond the Sonoran 

 habitat of megalops, must be regarded as un- 

 favorable. 



If variability in dorsal scales is related to 

 size, and has become definitely limited as a 

 physiological function of certain rows, it is 

 altogether possible, and to the reviewer it 

 seems probable, that the process of differentia- 

 tion into species has been much more complex 

 than the scheme so ingeniously developed by 

 Dr. Euthven, and that loss by dwarfing, and 

 gain, perhaps by reversion, have played their 

 respective parts over and again as species 

 have adapted themselves during their migra- 

 tions to unfavorable or favorable environ- 

 ments. 



This leads to the one of Dr. Euthven's con- 

 clusions which is most open to question, in 

 that his four lines of descent in Thamnophis 

 are traced back to northern Mexico as the 

 center of origin of the genus. 



As presented here there is incompleteness in 

 the theory, for it requires the existing forms 

 of garter snake to be left there, just as they 

 are, in a sort of cul de sac, from which there 

 is no further phyletic outlet. There is no 

 guide possible, even to speculation, as to a 

 common ancestral form, or as to the source 

 from which the genus was derived. 



Zoological geographers will be slow to be- 

 lieve that a group so largely dependent upon 

 water is likely to have originated in an arid 

 region, concerning which there is no reason to 

 suppose that in geologically recent times it 

 has been less dry than now. This general con- 

 sideration is of little moment in Dr. Euthven's 

 opinion, but certain other probabilities re- 

 main, to be less easily dismissed. 



From structure and life history there seems 

 good reason to believe that Thamnophis came 

 off from Tropidonotus, an almost cosmo- 

 politan genus, and one in all certainty much 

 older. Now Tropidonotus is distinctly not an 

 inhabitant of the Sonoran region, and makes 

 no approach to it nearer than the low gulf 

 coast of Mexico, and as an intruder up the 

 valley of the Eio Grande. There must be 

 significance in the absence of posterior 

 vertebral hypapophyses in all the genera of 

 colubrine snakes which with fair certainty 



