October 13, 1916] 



SCIENCE 



539 



Without insisting too much on the point, we 

 can not avoid noticing that this complex web 

 of theory is so exceedingly elastic as to be 

 capable of being fitted to a framework of cyto- 

 logieal fact, the converse of that for which it 

 was designed. Still, as some animals are found 

 to have no second heteroehromosome the sug- 

 gestion that such a body, when present, may be 

 inoperative might be offered in extenuation. 



Presently we meet, however, a fact which is 

 much more difficult to harmonize with the 

 theory, though constituting one of the most 

 novel and remarkable of the discoveries made 

 in the Drosophila work. Not only do the sex- 

 linked factors show no crossing over in the 

 male, but experimental breeding shows that in 

 the male there is no crossing over even of the 

 factors composing the other groups. Grossing 

 over, in fact, in Drosophila, turns out to he 

 exclusively a phenomenon of the germ cells of 

 females. This is a genetic discovery of the 

 first magnitude, whatever its ultimate signif- 

 icance, but the cytological interpretation of 

 crossing over must now bear a very consider- 

 able strain: for, on the one hand, though the 

 absence of crossing over in the sex-linked 

 characters had fitted well with the belief that 

 the sex-chromosome in the male was unpaired, 

 this chromosome is now admitted to be paired ; 

 and on the other hand the characters ascribed 

 to the chromosomes known to be paired turn 

 out to be equally unable to cross over in the 

 male. It is with some surprise that we find 

 neither in the book nor in the material previ- 

 ously published any coherent discussion of the 

 difficulties thus created. If further cytological 

 work shows that the chromosomes of the fe- 

 male twist and anastomose, but that those of 

 the male do not, the chromosomal theories of 

 heredity will receive a very remarkable sup- 

 port. Meanwhile on this part of the subject 

 there is little more to be said. 



Recombination then within the limits of a 

 linked group is regarded as a consequence of 

 crossing over, or the interchange of parts be- 

 tween one chromosome and its mate or homo- 

 logue. This conception, whether well- or ill- 

 founded, has led on to a further and very re- 



markable speculation. If the factors are 

 carried by the material of the chromosomes, 

 what more likely than that they, or rather the 

 particles severally bearing them, should be 

 arranged in a row, like a string of beads, along 

 the length of the chromosome ? The proportion 

 of cross-over gametes might thus give an indi- 

 cation of the actual relative positions of the 

 factors along the chromosome. On this in- 

 spiration, the intertwining of two strings of 

 beads providing always the mechanical anal- 

 ogy, the numbers in the experimental families 

 have been carefully studied. The percentage 

 of crossovers is taken to indicate the position 

 of the factors. Where there is no linkage, this 

 percentage is, of course, 50, all combinations 

 occurring in equal numbers. But if two 

 factors AB show 50 per cent, crossing over 

 and both A and B can severally be proved to 

 be coupled to a third factor C, then all three 

 may in reality be members of one linked group, 

 and the fact that in the case of one pair there 

 is 50 per cent, of crossing over may be a con- 

 sequence of the relative positions of these fac- 

 tors in a linear series. The amount of cross- 

 ing over can thus be interpreted as an indica- 

 tion of the relative positions of each factor in 

 such a series. Upon this follows the great 

 thesis of the book: that tbis series is in fact 

 a row of points along each of the four chromo- 

 somes, and that the redistributions or recom- 

 bination of characters can be correctly repre- 

 sented by strings of beads which twist together 

 in pairs, breaking and joining each other at 

 nodes. Whether this conception is sound or 

 not, we accept it as a gallant attempt to move 

 on. ~No other of equal promise has been offered 

 and we must observe its development with 

 cordiality and respect. 



Confronted with a theory of so much novelty 

 and importance, the reader's first desire is to 

 examine the details of the evidence from 

 which it has been deduced. A serious charge 

 lies against the book inasmuch as the mate- 

 rial for such an examination is not contained 

 in it. We are provided with a sketch — a vigor- 

 ous and impressionist sketch — of the facts as 

 the authors see them, but we want a much 



