502 



SCIENCE 



[N. S. Vol. XLI. No. 1057 



Tinder corresponding circumstances is 1:2:1; 

 that is, there should be one pair of boys, to two 

 mixed pairs, to one pair of girls. In other 

 words, if the members of a pair of twins al- 

 ways developed from separate ova, we should 

 expect to find twice as many pairs whose mem- 

 bers differ in sex, as there are pairs of girls, 

 or pairs of boys. I have been able to think of 

 no factor which may reasonably be supposed 

 to be acting in a constant direction to alter 

 this ratio. 



I have undertaken to compare with this 

 hypothetical ratio the ratio found among 

 births of twins in this country. My data 

 number 3,334 twin births which occurred in 

 the states of Connecticut, Maine and Ver- 

 mont during the years 1899 to 1912. Of this 

 number 1,118 are pairs of boys, 1,193 are boy 

 and girl, and 1,023 are pairs of girls. This 

 is almost a 1:1:1 ratio, showing the effect, 

 however, of the predominance of male births. 

 There is obviously a large excess of pairs sim- 

 ilar in sex over what is to be expected on the 

 supposition that twins originate in all cases 

 from separate ova, an excess of more than 500 

 pairs of boys, and almost 500 pairs of girls. 



This seems to point towards the conclusion 

 that twins may originate from a single fer- 

 tilized ovum. In the light of present knowl- 

 edge this certainly is a possible explanation of 

 the statistics. If the figures given will bear 

 this interpretation, we may say that less than 

 haK (44.3 per cent.) of the tw^in births of sim- 

 ilar sex, or less than one third (28.4 per cent.) 

 of all twins, originate from one ovum, while 

 slightly more than half (55.7 per cent.) of 

 those of similar sex have developed simul- 

 taneously from two separate ova. 



Margaret V. Cobb 

 Palls Church, Va. 



naturalist's directory 



To THE Editor of Science: As you have 

 given liberal space to criticize the book, you 

 will doubtless be willing to give space in 

 which I can explain the matter. 



In the first place this book has not been 

 issued for some eight years, and in getting out 



the new edition I decided that not a single 

 name would be included unless I had a request 

 that the name should be included from each 

 party. If you find that there are a good many 

 naturalists omitted from the directory, it was 

 because they were too busy, or more likely too 

 careless of such matters to take time to return 

 the blanks which I sent them. Every nat- 

 uralist of any consequence, and a great many 

 collectors, received three notices each and none 

 of the names were included in the book unless 

 they replied. 



Since getting out the work some of these 

 noted scientists have taken time to write three 

 or four criticisms of the book, while they would 

 not take time before publication to even sign 

 their names to the blanks I sent them. There 

 are a few typographical errors in the book as 

 there are bound to be in any work of this kind, 

 and the transposition of two or three entries, 

 to which you have taken great pains to call 

 attention, was caused by the misplacement of 

 one or two linotype slugs. 



It is my intention to get out another edition 

 of the Naturalist's Directory in a year from 

 now, and I hope naturalists, generally, will 

 be as free with their assistance in bringing the 

 new edition up to date, as they have been in 

 criticizing the edition just published. 



S. E. Oassino 



Salem, Mass. 



SCIENTIFIC BOOKS 



Die Variolation im acMzehnten Jahrhundert. 

 Ein historischer Beitrag zur Immuniidtsfor- 

 schung. By Arnold 0. Klebs. Giessen, 

 A. Topehnann. 1914. 8vo. Pp. 78. 

 Few physicians know that throughout the 

 entire eighteenth century, and before Jenner's 

 time, there was a vast wave of experimental 

 research in the problem of preventive inocula- 

 tion against disease, now almost forgotten. 

 Starting in 1713, it passed into a period of 

 twenty years' stagnation about 1727, with a 

 revival in 1746 and a truly scientific phase 

 during 1764-98. When a bibliography of some 

 600 titles, by the author of the above mono- 

 graph, was shown to a highly educated physi- 



