148 



EVOLUTION AND ANIMAL LIFE 



such numerical or, as called by him, meristic variations, must be looked 

 on as different in kind from the substantive variations — those capable 

 of perfect merging from one condition to another — in other words, 

 practically incapable of quantitative measurements. These meristic 

 variations are called discontinuous by Bateson. Typical examples are 

 the variation in the number of the costal wing hooks in bees and ants, 

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

 metathoracic tactile hairs in biting bird lice, etc. But when one stops 

 to consider the fact that in all these cases variation could hardly occur 



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Fig. 88. — Variations in pattern of wings of Peronea cristana. (After Clark.) 



by any steps less than those of one hook or one spine or one hair, that 

 a half hook or half antennal segment is inconceivable, serious doubts as 

 to the validity of Bateson's classification of variations as continuous 

 and discontinuous will certainly result. The doubt is strengthened 

 by the difficulty of a clean classification presented by such cases as that 

 of Hippodaviia convergens (Figs. 72, 73 and 74). Here we have a 

 substantive variation in pattern, appearing, however, in such a way as 

 to demand numerical, i. e., meristic, expression. One specimen has 

 nine elytral spots, another ten, another eleven, and so on; the whole 

 range is indeed from naught to eighteen, with every number between 

 represented, each by various combinations of spots. 



" But it is conceivable, and indeed is really the case among our 

 specimens, that these spots might be either of normal size, or of any 

 lesser size down to the limits of \dsibiUty. Some of the spots are of 

 the diameter of pin points, some of the pin shaft, and some of pin 

 heads. There is perfect gradation and continuity in this variation. 



