ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 27 



chromosomes may be regarded as compound, made up of units corre- 

 sponding with the separate chromosomes of Tthysia zonaria. 



Another aspect of the individuality question is touched by the 

 phenomena of the maturation divisions of the hybrids. Although the 

 majority of the chromosomes fail to pair in synapsis, it is certain that some 

 of them do so. Perhaps the chromosomes are not individuals in the sense 

 of being indivisible units, but are composed of units, and perhaps the pair- 

 ing in synapsis is due to some affinity between chromosomes made up of 

 similar components. If there is any truth in this view, it may give some clue 

 to the baffling problem of interspecific sterility. The hirtaria-zonaria 

 hybrids are completely sterile. If the haploid set of 56 chromosomes of 

 zonaria consists on the whole of the same units as the 14 haploid chromo- 

 somes of hirtaria, combined in different ways, the sterility between the two 

 nearly allied species may have arisen simply from a difference of group- 

 ing. Since the units are grouped differently, the chromosomes cannot 

 pair properly in synapsis, and this may be the cause of the sterility. 

 Doncaster also discusses the possible cause of the fact that only males 

 are produced from the cross zonaria 9 and hirtaria £ . 



Variability of Tiger Moth.* — Kurt Smollian has made a careful study 

 of the variability of Arctia caja, the garden tiger-moth. There is almost 

 no variability in shape. Changes in the shape of the wings are associated 

 with changes in venation, and these are almost exclusively pathological. 

 The males are on an average smaller than the females, and their range 

 of variation in size is less. The greater the inhibition in development, 

 the smaller the imago. Inhibitions of growth are due to. extremes in 

 the conditions of life, e.g. too high or too low temperature, too much 

 or two little humidity, insufficient or unsuitable food, and insufficient 

 light. Influences which increase the vital activity of the larval and 

 pupal stages increase the size of the imago. 



As to colorations and markings, variants induced by frost and 

 great heat are individual aberrations. Those induced by low tempera- 

 ture but not frost, are mostly reversionary, especially as regards the 

 posterior wings. Those induced by warmth, but not great heat, corre- 

 spond to local southern races. Frost and great heat induce discontinuous 

 variations ; less extreme temperatures induce fluctuations. Modifications 

 of the caterpillar do not as such pass on to the imago. The females 

 are more susceptible than the males. There is evidence of a supero- 

 inferior direction of development in the fore-wings, of an infero-superior 

 direction in the hind-wings, and of an antero-posterior development in 

 both. Each character varies per se in its own direction. There is a 

 well-marked correlation between the coloration of the thorax and that 

 of the anterior wings, between that of the abdomen and that of the 

 posterior wings. The coloration of the imago is ontogenetically quite 

 independent of that of the larva. 



The scales of the posterior wings are broader and have sharper 

 processes than those on the anterior wings, and have more hairlike 

 structures on the root area. These characters are ontogenetically (and 



* Jen. Zeitschr. Naturw., 1. (1913) pp. 411-600 (6 pis. and 64 figs). 



