224 COI.ORATION IN I^^PTINOTARSA. 



and complete orthogenetic series. These elytral color patterns are the most 

 important of the specific differentials, are perfectly stable characters in each 

 species, and are as reliable as indicators of relationship and evolution as any 

 other characters we may examine. Let us ignore for the moment the ques- 

 tion of the relationship of the species and the order and evidence of their evo- 

 lution, and concentrate our attention upon the very plain case of orthogenesis 

 presented in the effort to discover why this orthogenesis exists. 



In the construction of plates 23 and 24 the species were not selected to 

 illustrate orthogenesis, but every species of the genus is represented save 

 two — L. rubiginosa, a unicolorous bright red form, and L. zetterstedti, a 

 brown-banded species. Both these excluded species stand wide apart from the 

 rest of the genus, and are more nearly related to species of Zygogramma. 

 They are in reality the end results of lines of modification in that genus, and 

 show the same phenomena of orthogenesis. The fact that all the species of 

 the genus show this phenomenon is of itself adequate proof without further 

 argument of the formation of species in definite directions. (I use the word 

 orthogenesis in the sense in which it was first used, and not in that employed 

 by Eimer.) However, upon whatever basis we are content to explain this 

 condition, the fact remains that the species as we now recognize them have 

 elytral color patterns of such a character that their arrangement in an ortho- 

 genetic system is accomplished with ease. This fact might be explained upon 

 the basis of Eimer's orthogenesis, by an internal perfecting tendency irresist- 

 ibly forcing the species in certain directions, by natural selection through an 

 elimination of all variations excepting those along the lines which were of 

 utility to the species, by the neo-Lamarckian factors, or by mutation. And 

 yet, with each or all of these hypotheses, we are far from any real elucidation 

 of the question as to why the establishment of species along certain lines has 

 taken place. A possible explanation of the problem is to be found in the 

 phenomena of development, and it is quite possible that the final interpreta- 

 tion will rest upon the laws of ontogenetic development and physiology alone. 



In the ontogeny of elytral coloration and in the coloration of adults it has 

 been shown that color develops in stripes, bands, or spots, and that the evolu- 

 tion of patterns has been orthogenetic, by means of various combinations of 

 these bands, stripes, or spots. If now we can arrive at some explanation of 

 these fundamental color elements, we shall be better able to understand the 

 whole problem. In order to do this we shall first have to consider some of 

 the phenomena in the development of the wings. 



The wings of these beetles and of all insects arise from thickenings upon 

 the sides of the last two thoracic segments, followed by invagination, and 

 later by evagination, to form the wing fundament. In Leptinotarsa they 

 always arise beneath the wing spots, which, as I have shown, are homo- 

 dynamous with the spiracular spots. The wing fundament consists at first of 

 a minute area in which the cells are more closely crowded together than in the 



