VARIATION AND MUTATION 145 



that when the adults issue, the variations to be found in any of their 

 parts may with complete confidence be ascribed to prenatal influences, 

 to intrinsic causes. They are purely blastogenic. Similarly, the con- 

 ditions of life of the developing individuals of all the other social 

 insects, the termites, ants, and social wasps, are practically identical. 



"The variations, therefore, in the color pattern of Diabrotica (Fig. 

 75), Hippodamia (Figs. 72, 73 and 74), and Vespa (Fig. 76) (insects 

 of complete metamorphosis with all adult external structures never 

 exposed to outside conditions until in definite unchangeable condition), 

 are congenital variations. Of the same nature are also the structural 

 variations in the character of the venation and the number of wing 

 hooks in the honey bee (see Fig. 94). But the variations in the pat- 

 tern of the prothorax of the flower bug (Fig. 77), and in the number 

 of spines on the tibiae of the red-legged locust (Fig. 78), and the cicada 

 (Fig. 79), may be in part acquired. In these latter cases the insects, 

 not having a complete metamorphosis, have during their immature 

 life these color and structural characters in formative condition, and 

 to some extent in use. They are therefore exposed to the continuous 

 influence of their environment." 



It might be thought that we could determine whether varia- 

 tions are congenital or acquired in cases in which we are thor- 

 oughly acquainted with the character of the environment or ex- 

 trinsic influences which have surrounded the individuals during 

 their development. In experimental cases we can control this 

 environment and make it identical for all of a given lot of 

 individuals, or measurably varying for different lots. Then 

 by comparison we can determine what characteristics still vary 

 among those individuals exposed to identical environment- 

 these variations should be congenital and what new kinds of 

 variations appear in those individuals exposed to different en- 

 vironments these should be acquired variations. This has 

 been done experimentally for silkworm moths. By varying 

 the food supply, etc., marked variations have been produced in 

 the size of larvae and moths, weight of silken cocoons, duration 

 of larval stages (instars). These variations are manifestly 

 acquired, and wherever in nature simple variations in dimen- 

 sions are found among individuals of a species, this is due un- 

 doubtedly, to greater or lesser extent, to differing conditions of 

 nutrition. But we know well that a practically identical food 

 supply given to domesticated animals or human beings can 



