Januabt 25, 1907] 



SCIENCE 



151 



the hills of ancient Eome in the number and 

 variety of Vultures that I have been able to 

 discern. 



With Dr. Allen's closing statement that the 

 first species rule ' has only here and there a 

 disciple' or that it has ever been generally 

 abandoned in practise so far as ornithology is 

 concerned, I beg to differ. 



The interviews and correspondence that I 

 have had since my paper was published show 

 that the adoption of the first species rule as 

 there outlined meets with very general ap- 

 proval among vertebrate zoologists as well as 

 entomologists, while botanists, as is well 

 known, have long practised it. 



One prominent entomologist in a recent 

 publication hopes that it may be incorporated 

 in the International Code at an early date, 

 while one of the foremost zoologists of 

 America writes me that " elimination is ab- 

 solutely dead and ought not to be revived in 

 any code or thought of in any connection." 



A thorough discussion of this subject is de- 

 sirable, but really, my friend Dr. Allen and I 

 are of nearly the same mind on the question. 

 He says at the beginning of his article : " I 

 have always conceded that this [i. e., the first 

 species principle] would be the ideal method 

 if we were at the threshold of our work * * * 

 and my opposition to it has been * * * that 

 to adopt it now would introduce serious con- 

 fusion into nomenclature." This was exactly 

 my view, and when upon investigation I found 

 that serious confusion (so far as birds are 

 concerned) would not ensue, I thought that 

 there were no further grounds for objection. 

 The other objections that have occurred to 

 Dr. Allen in the later pages of his paper I 

 have tried to dispel. 



At the present time I feel more sure than 

 ever that the zoological code that adopts the 

 first species rule (excepting in relation to 

 Linnseus) will be setting an example which 

 will in a few years be followed by vertebrate 

 zoologists in general and, with a possible 

 further limitation, by invertebrate zoologists 

 as well. 



Wither Stone 



Academy of Natueal Sciences 

 OF Philadelphia 



SPECIAL ARTICLES 



ON A CASE OF REVERSION INDUCED BY CROSS- 

 BREEDING AND ITS fixation' 



Perhaps the most important extension 

 which has been made of the law of heredity 

 originally discovered by Gregor Mendel con- 

 sists in the demonstration (chiefly by Cuenot 

 and Bateson) that certain characters are pro- 

 duced only when two or more separately 

 heritable factors are present together. Such 

 a character does not conform with the simple 

 Mendelian laws of inheritance, but its factors 

 do. Herein lies the key to the explanation of 

 so-called heterozygous characters and to the 

 practical process of their fixation. This same 

 principle serves to explain also atavism or 

 reversion, and the process by which rever- 

 sionary characters may be fixed. 



When pure-bred black guinea-pigs are mated 

 with red ones, only black offspring are, as a 

 rule, obtained. The hairs of the offspring do 

 indeed contain some red pig-ment, but the black 

 pigment is so much darker that it largely, ob- 

 scures the red. In other words, black behaves 

 as an ordinary Mendelian dominant. In the 

 next generation black and red segregate in 

 ordinary Mendelian fashion, and the young 

 produced are in the usual proportions, three 

 black to one red. All black races behave alike 

 in crosses with the same red individual, but 

 among the reds individual differences exist. 

 Some, instead of behaving like Mendelian re- 

 cessives, produce in crosses with a black race 

 a third apparently new condition, but in 

 reality a very old one, the agouti type of coat 

 found in all wild guinea-pigs, as well as in 

 wild rats, mice, squirrels and other rodents. 

 In this type of coat red pigment alone is found 

 in a conspicuous band near the tip of each 

 hair, while the rest of the hair bears black 

 pigment. The result is a brownish or grayish 

 ticked or grizzled coat, doubtless inconspicu- 

 ous and so protective in many natural situa- 

 tions. Some red individuals produce the 

 reversion in half of their young by black 

 mates, some in all, and others, as we have seen, 

 in none, this last condition being the com- 

 monest of the three. It is evident that the 



^ Publislied by permission of the Carnegie In- 

 stitution of Washington. 



