. Carnegie Institution of Washington publication. ii4 VARIATION IN IvEPTINOTARSA. resent a line of development in which the three species are steps in a certain direction of evolution. It would be difficult to invent a hypothesis to account for this case other than the one advanced above that would not contain obvious absurdities. The species multitceniata, oblongata, intermedia, melanthorax, rubicunda, and decemlineata, which are very closely allied forms, are all, excepting decemlineata, confined entirely in their distribution to the Mexican table-lands. This is suggestive. As I have shown i
. Carnegie Institution of Washington publication. ii4 VARIATION IN IvEPTINOTARSA. resent a line of development in which the three species are steps in a certain direction of evolution. It would be difficult to invent a hypothesis to account for this case other than the one advanced above that would not contain obvious absurdities. The species multitceniata, oblongata, intermedia, melanthorax, rubicunda, and decemlineata, which are very closely allied forms, are all, excepting decemlineata, confined entirely in their distribution to the Mexican table-lands. This is suggestive. As I have shown in the first chapter, we may regard mid- titceniata as the central species in this group, variable, plastic, and able to live in diverse habitats. It has spread over a wide area, and as it has been dissem- inated new species have been produced. In all their variations this series of species shows the same kind and direction of modification; they differ, how- ever, in the extent of their variation and in the portion of the possible scale thereof which they exhibit. Thus, multitceniata, intermedia, and decem- OBLONQATA RUBICUNDA DECEMLINEATA t INTERMEDIA MULTIT/ENIATA. MELANOTHORAX SIQNATICOLLIS ANQUSTOVITTATA DIVERSA HYPOTHETICAL ANCESTOR Text-figure 4.—Scheme of phylogenetic development of the hneata group, based upon the data of distribution and variation. lineata, form a series from north to south over the continent, which show not only geographically arranged steps, but steps also in the variations presented; from multitceniata, with variation entirely in the melanic half of the possible series, through intermedia to decemlineata, with its variations in the middle and albinic end of the possible variation series. Thus these three species,and to the south,in like manner, oblongata, show differentiation in one direction, both geographically and in their fluctuating variations. Melano- thorax and rubicunda, which live in the same habitat with multitceniata, rep- resent more lo
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