108 



SCIENCE 



[N. S. Vol. XXV. No. 629 



In serpents, such variations as the presence 

 or absence of certain head plates, or of a pair 

 of dorsal rows of scales, are fairly common in 

 many genera, but as a rule they do not trans- 

 gress the obvious limits of specific variation, 

 and unless combined with other differences 

 they are not regarded as deserving of a name. 

 Nevertheless, when they do transgress they 

 fall within our definition of a mutation, for 

 these characters are the indivisible units of 

 repetitive series, and between their presence 

 or absence there can be no intergradation. 

 Among the species and subspecies enumerated 

 by Cope, there are thirteen such cases which 

 might possibly be allowed as mutations. But 

 even granting them to be such, they seem to 

 have failed signally in giving rise to new 

 species, for nine of them are known only from 

 the one type specimen each, and of the tenth, 

 two examples only were collected more than 

 twenty years ago, at the same time, in a well- 

 settled part of Texas. The remaining three 

 cases, of more or less established forms, have 

 some claim to consideration. They are these: 



The genus Storeria consists of three species, 

 two of which, S. dekayi with seventeen rows 

 of dorsal scales, and 8. occipito-maculata with 

 fifteen, occupy practically the same range from 

 Vera Cruz north over most of the Austrori- 

 parian and eastern regions. There are slight 

 color differences, fairly constant, but the dif- 

 ference in scale rows seems to be entirely so, 

 and all herpetologists admit their specific dis- 

 tinctness. As there can be no gradation be- 

 tween fifteen and seventeen scale rows, which 

 vary always in pairs, one or the other of these 

 species, probably S. occipito-maculata, seems 

 to have arisen from the other by a process 

 which might be called mutation. It may be 

 allowed that the differential characters are not 

 adaptive. 



In exactly the same way Virginia elegans, 

 occupying a limited westei'n portion of the 

 range of V. valerioe, differs from it in having 

 two more scale rows. 



Finally, Eutcenia elegans atrata (^E. in- 

 fernalis vidua Cope) appears to be an offshoot 

 of E. elegans, presenting a quite distinct color 

 pattern and a tendency to a reduction of scale 

 rows. According to Van Denburgh they are 



found only on the coast slope of the jyeninsula 

 of San Francisco, and the examples I have 

 seen were collected promiscuously with typical 

 E. elegans. The mutation in this case would 

 lie in the distribution of color, for the reduc- 

 tion in scale rows is not fully constant. 

 Whether they breed true is not known, but 

 their scarcity renders it doubtful. 



But we are now close to a mere matter of 

 names, for in two, at least, of these cases 

 variation and mutation approach each other 

 so nearly that they come under the same defi- 

 nition, for the addition or subtraction of a 

 pair of scale rows represents the lowest pos- 

 sible term in a variatien series, and the name 

 given to it is largely a matter of choice; yet 

 beyond these cases no other evidence for the 

 origin of specific characters by mutation is 

 yielded by the examination. The conclusion 

 is near to that of Dr. Merriam. 



The value of the experimental method is 

 not questioned by the doubt whether theoret- 

 ical interpretation of the behavior of ' unit 

 characters ' in the germ plasm has yet reached 

 a stage of certainty sufficient to stand over 

 against the body of evidence contributed by 

 the comparative method, as to the minor role 

 of mutations in specific development in verte- 

 brates. 



That mutants occur in feral animals is 

 doubtless true, even much more widely than 

 the cases of melanism and albinism cited by 

 Professor Davenport, but it does not yet seem 

 necessary to modify the opinion not long since 

 expressed by me elsewhere" — " In so far as its 

 occurrence under nature is concerned, every 

 zoologist who has worked over many genera 

 for purposes of taxonomy will probably admit 

 that many of his most perplexing anomalies, 

 which occur now and then as one or a few 

 individuals which can not be exactly placed, 

 are in the nature of mutations, but few, I 

 imagine, will be disposed to allow that they 

 find evidence that these are inhenited. . . . 

 there is little evidence that they have been 

 starting points of new species." 



Arthur Erwin Brown 



The Zoologigai, Gardens, 

 Philadelphia 



' ' Theories of Evolution since Darwin,' 1906. 



