146 



EVOLUTION AND ANIMAL LIFE 



never make all the individuals of a single brood or family of the 

 same size. Part of the dimensional variation is due, therefore, 

 to congenital causes. In a beehive, the condition of temper- 

 ature, humidity, and food supply are practically identical for 

 all the developing bees, and yet bees born of eggs laid by a 



single queen, reared at the same time in 

 the same hive, vary largely in such easily 

 determinable and important matters as 

 venation of the wings, number of hooks 

 used in holding the two wings of one side 

 together, color pattern, etc. Undoubtedly, 

 these variations are strictly congenital, hence 

 inheritable, and therefore of a character to 

 serve as a basis for species change. 



With regard to examples of continuous 

 and discontinuous variations, we take the 

 FIG. 84. Head of deer following from the paper on "Variation in 



with one prong of 

 horns markedly dif- 

 ferent from the other. 



(After Bateson.) 



continuous variations we reter to those 

 variations mentioned above, variously called 



fluctuating, individual, etc., which are present in any series of indi- 

 viduals of a species, and which cluster about the modal or most 

 abundantly represented forms of the species, as would be expected 

 from the law of error (law of probabilities) dis- 

 cussed above. 



"Morgan, in 'Evolution and Adaptation/ ob- 

 jects to the use of ' continuous ' as a descriptive 

 name for these variations, on the ground that the 

 word suggests persistence or continuity through 

 successive generations. It seems to us, however, 

 that the name is an apt one, if ' continuous ' be 

 taken to mean that the occurring variations in 

 any (sufficiently large) set of individuals form a 

 continuous series, the extremes being connected 

 or immediately merging into each other by a 

 series of small gradatory steps. By 'discontinuous' variation, we 

 would mean, in contrast to continuous, such considerable and radical 

 changes as have been variously called single variations, sports, saltations, 

 mutations, etc.; that is, variations which are not members of graded 

 series and do not group themselves in orderly manner about the modal 



FIG. 85. Turtle with 

 two heads. (After 

 Bateson.) 



