234 The Mechanism of Evolution in Leptinotaksa 



gave body-index. The breadth of the elytra would have been useful, but too 

 much difficulty was encountered in eliminating obvious sources of error in the 

 measurement. This index is a good indicator of the general " body-forms " so 

 often useful in taxonomic work and a real characteristic of the mass. On 

 plate 36 are shown some of the different forms of body which this species 

 exhibits, and in the main can be grouped into three assemblages, of which the 

 muUilineata form represents one extreme, some of the melanothorax types the 

 other, and the typical muUitmniata the intermediate. Any one of these types is 

 " variable " in its form as found in nature and even in cultures that have been 

 purified with reference to the color and pattern differentials. My problem was 

 to purify one of these forms if possible, and then analyze or produce in it 

 " variations," in the hope of being able to gain some clue to their causation and 

 significance. I took the muUilineata type as the start in the problem as being 

 the simplest, and the problem was to obtain a constant and pure muUilineata 

 type. 



In the years 1904, 1905, and 1906 many preliminary tests were made, so that 

 at the end of 1906 I had 6 lines of this material that were thought usable as a 

 basis for the study. In the course of breeding experiments with this species I 

 had found and identified three sets of factors that were productive of form and 

 form changes in the species L. multitcBniata, and I had been able to prove the 

 reality of these by the repeated transference of them to the species L. decem- 

 lineata and L. oblongata, replacing in the normal form the " body-form factors " 

 from the strains of L. muUitmniata. The materials which I had at the end of 

 1906 were 6 races with the form-factors so purified by being repeatedly extracted 

 and purified in genetic operations that the strain was highly constant in both 

 size and shape, ranging well within the index limits of 3.500 and 3.000, and 

 there also had been an effort to reduce the complications in color and other 

 characters as far as was possible, so as to give a homogeneous-acting biotypic 

 form. The strains were now in the condition that would have admitted being 

 characterized as highly refined " biotypes " showing only the usual expected 

 " fluctuation " about an average value. 



In 1907 I carried 5 of these through 3 breeding-periods, or 4 generations in 

 all, and of the 5 all showed the same results in that they were constant to the 

 type of the line, with the usual range of divergences in the character chosen for 

 the examination. All would have been characterized as constant stocks, and 

 the problem became, what was the cause of the range presented by the index? 



The sixth line of the original series at the beginning of 1907 was composed 

 of 63 males and 66 females that were the progeny of a single pair of parents 

 and emerged from the hibernation in the winter of 1906-07, and were an ex- 

 tremely uniform lot of individuals. These were divided at random into 4 lots 

 of 15 each, and within each of these lots 5 pairs were mated at random, and these 

 bred out through 3 generations in March, April, and May 1907, and entered 

 hibernation in the early days of June. The progeny from these matings were 

 imiform and like those from the 5 main stocks reared during the same time and 

 imder the same conditions; 17 of the pairs gave progeny and the range in the 

 index of these, as well as the same character in one of the 5 " stock " lines, is 

 shown in table 38 for the first generation of 1907 and in table 39 for the second 

 generation of 1907. 



