254 The Mechanism of Evolution" in Leptinotaesa 



and inbred as division B, division A being the normal culture. In F4 the normal 

 group (A) from a routine mating gave 36 males and 45 females, while all of the 

 progeny of the divergent B group w^ere entirely of the normal type arrayed in 

 classes c, d, and e, there being 14 males and 17 females. From these F4 groups 

 matings were made as before from A and B, and in both normal progeny resulted 

 in A, 94 males and 94 females, and in B, 23 males and 22 females. The B group 

 was not carried any longer, while the A group continued from the usual mating, 

 giving Fg all normal, 292 males and 282 females. From these a mating of the 

 ordinary type gave F^ with two groups, the normal (A) and a divergent one (C) 

 well outside the normal range. In the first there were 164 males and 166 

 females, and in the second, 1 male and 2 females. A mating of the first group 

 gave normal progeny in Fg, and again in Fg, when the A line was terminated. 

 The C group gave a progeny in Fg all of primary biotype 12, 29 males and 30 

 females, distributed in classes a, b, and c. A mating from these gave Fg, again all 

 of type 13, 67 males and 76 females, and from a mating taken from these a 

 progeny was reared in F^q, again all of type 12, 156 males and 166 females. At 

 the end of F^^ this line was also discontinued. 



This culture of biotype 9 shows two common types of behavior that are found 

 in these groups, namely, individuals that appear beyond the normal range but 

 which, when inbred, give the normal population, and a group (c) beyond the nor- 

 mal range and without intergrades, which, when inbred, give individuals true to 

 their own type, which continued to breed true to that type. That is, in the lan- 

 guage of the adherents of the hypothesis of pure lines, biotype 9 had " mutated," 

 giving biotype 12. The individuals of the group c, however, were the last to 

 emerge ; indeed, it was thought that all that were going to emerge had done so 

 and the cages were therefore set aside before being cleared for the next mating. 

 In this period of over a week they dried completely and were often heated to 

 nearly 100°F. The modified group (c) emerged after this treatment. Whether 

 the treatment and the divergent group are cause and effect is a matter of opinion. 

 It seems plausible to think they are, but the test is too crude to be more than a 

 plausibility. This occurrence does, however, show one point of importance — 

 the delicacy of balance of some of these groups and the ease with which incident 

 forces may possibly give rise to sudden changes, " mutations," to other biotypes. 



Each one of the biotypes shown in figure 35 has been reared in this same 

 manner many times with uniform results. They are groups which, under uni- 

 form conditions, are permanent and stable even in unselected group cultures. 

 Under uniform conditions arid isolated they remain true to type, fluctuating 

 within a fairly normal and fixed range, but in all of them there is considerable 

 unstability manifested in the presence of extreme conditions or of intense 

 stimuli, or even of rapidly varying conditions, giving as a result sudden changes 

 of a few individuals to other biotype groups. 



In this array there are also differences as regards their stability. Biotype 1 

 is highly stable and difficult of disarrangement, while biotype 7 is the least stable 

 of all. Biotypes 5, 6, and 10 are fairly stable, while between 9, 11 and 12, and 3, 

 3, and 4 sudden transmutations are common and easily produced, and the con- 

 viction is soon forced upon the observer that these groups, although distinct and 

 stable under constant conditions, realities in every respect, would nevertheless 

 under natural conditions present a never-ending series of transmutations as the 

 series of external forces passed over the population. 



