542 



SCIENCE 



[N. S. Vol. XLIV. No. 1137 



two X chromosomes. But for this it would 

 have been male, for it is fertilized by the 

 sperm normally destined to males. Since also 

 all the dominant sex-linked factors possessed 

 by normal males are borne by the sperm nor- 

 mally destined to daughters, the sperm that 

 the exceptional daughter receives is recessive, 

 and therefore these daughters are matroklinous. 

 It follows as a corollary from this argument 

 that fertilization might take place between ova 

 bearing no X at all and a sperm bearing X, 

 and it is said that such a class has been actu- 

 ally recognized as consisting of sterile males. 

 Once the matroklinous daughter has appeared, 

 by breeding from her, a complex variety of 

 consequences may be expected, all deducible 

 from the a priori analysis. In the breeding 

 experiments, apart from certain numerical 

 aberrations still unexplained, these have now 

 all been realized experimentally. 



Cytologically also the expected appearances 

 have been found — in the sense that egg cells 

 of the " exceptional " females have been seen 

 to contain three instead of two of the chromo- 

 somes which the authors now agree are the 

 heterochromosomes. Moreover, from an XXT 

 female it should be possible to breed an XTY 

 male and the two in combination may lead to 

 forms with XXYY, and figures are given 

 showing that these also have been produced 

 and cytologically demonstrated. No one can 

 doubt that this is a very fine achievement. 

 Though still sceptical as to the adequacy of 

 the theory of cross-overs and especially of the 

 soundness of the arguments by which the fac- 

 tors are assigned to serial positions in the 

 chromosomes, it is difficult to see how we can 

 deny that the sex-linked characters have some 

 very special relation to the sex-chromosomes. 



In our present ignorance of the nature of 

 life we cannot distinguish cause and effect in 

 these phenomena and it is not possible to at- 

 tach any satisfactory meaning to the expres- 

 sion that the sex-linked factors are " carried " 

 by a chromosome, but if any one wishes to de- 

 scribe the association of the phenomena in 

 that way there is nothing to forbid him. The 

 properties of living things are in some way 

 attached to a material basis, perhaps in 



some special degree to nuclear chromatin ; and 

 yet it is inconceivable that particles of chroma- 

 tin or of any other substance, however com- 

 plex, can possess those powers which must be 

 assigned to our factors or gens. The suppo- 

 sition that particles of chromatin, indistin- 

 guishable from each other and indeed almost 

 homogeneous under any known test, can by 

 their material nature confer all the proper- 

 ties of life surpasses the range of even the 

 most convinced materialism. Hence it may 

 well be imagined that even if cytologists de- 

 cide that in synapsis there is no anastomosis 

 and no transference of material, the effective 

 transference of the gens may occur. The 

 transference may be one of " charges." Per- 

 haps even we might profitably consider 

 whether the chromosomes may not be thrown 

 up, and the gens grouped along their lines 

 by the interplay of the same forces. 



Though as must frankly be admitted the 

 Drosophila work is on the whole favorable, 

 and in certain respects strongly favorable, to 

 the view that all segregation is effected at the 

 reduction division, it may be well to remind the 

 workers in this field of the phenomena which 

 are inconsistent with that conception. There 

 are, of course, the old difficulties that if the 

 chromosomes play this prerogative part we 

 should expect some broad consistency between 

 their differentiation and that of the forms of 

 life, and we should not anticipate that they 

 would be capable of great irregularities of 

 number and behavior. But apart from these 

 there remain the perfectly authenticated in- 

 stances not merely of somatic differentiation in 

 regard to Mendelian characters, but the whole 

 range of bud-sports and chimeras of various 

 kinds, and lastly the indubitable evidence that 

 the male and female sides of the same plant 

 may have distinct genetic properties. Such 

 facts, to be sure, are no indication as to the 

 powers of chromosomes, but they are a strong 

 indication that the reduction process is not 

 the only moment at which segregation may be 

 effected. Presumably the advocates of chromo- 

 somal views would admit that these are ex- 

 ceptions, but still they are exceptions of a 

 most significant kind. Conceivably we may 



