VARIATION AND MUTATION 



149 



And even in such cases as variations in spines and hairs, this gradation 

 might exist : and indeed it does. Although in our consideration of the 

 variation in the number of the tibial spines of the locust and cicada 

 and in the number of the tactile hairs of the bird lice, we have referred 

 to these variations only numerically, i. e., meristically, as a matter of 

 fact there are obvious differences in the length, i. e., size, of the spines 

 and hairs, so that it would be wholly fair to break down the unit differ- 

 ences and speak of differences by one quarter, one third, and two thirds 

 of a spine. For the tibial spines of the locust, we have actually re- 

 corded the conditions in the form of frac- 

 tions. But in the case of a hook or an 

 antennal or a tarsal segment it is a unit or 

 nothing. 



^^To our mind, the distinction between 

 substantive and meristic variation is not 

 at all equivalent to a distinction between 

 continuous and discontinuous variation. 

 It is a distinction between two categories 

 of variation only in that one category in- 

 cludes such conditions as permit more 

 readily of extremely slight, nearly insensi- 

 ble, practically unmeasurable differences, 

 as those of pattern or shape or extent, 

 while the other category includes partic- 

 ularly conditions in which any variation 

 must of necessity be fairly obvious, and 

 usually capable of numerical expression. 



" But we believe, nevertheless, that variations really discontinuous 

 occur among insects. For example, the occurrence of interpolated, 

 wholly new, and complete cells (determined by the presence of new 

 cross veins or branches of longitudinal veins) in the fore and hind 

 wings of drone honey bees (Figs. 93 to 96) and the occurrence of 

 curious malformations of venation among drone bees must be looked 

 on as sports or truly discontinuous variations. The regular occurrence 

 of a four-segmented foot, perfectly complete, functional in those numer- 

 ous specimens of cockroach (Fig. 89), in which natural regeneration 

 has taken place, may be looked on as an example of discontinuous 

 variation. Although no difference in tarsal segments less than that of 

 one is conceivable, it is quite conceivable that the foot with one fewer 

 than the normal number might be in such condition that it would be 

 obviously a five-segmented foot with one segment dropped out: in 

 11 



FIG. 89. Cockroach, showing 

 varying number of tarsal seg- 

 ments in legs. (After Kellogg 

 and Bell.) 



