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 PHYSIOLOGIC SPECIES

mutations that will always attract first attention involve such morphologic structures as we have considered in the previous section. On the other hand, there are three groups of physiologic characters in the genus Cynips which allow additional insight into the nature of species. These physiologic qualities are to be observed in the form of the gall, the life histories, and the host relations of these gall wasps.

The galls produced by the Cynipidae are, of course, plant tissue, but their form and structure depend largely upon the nature of the gall-producing stimulus which the insect puts into the plant. The precise source and nature of this stimulus is not known, but there seems every reason for believing that it partakes of the nature of an enzyme or hormone produced by some particular insect structure. To a limited extent the form of the gall does depend upon the plant tissue involved, for the possible transformations, as we have already pointed out (Kinsey 1923:21), are more restricted in certain tissues; but beyond that the form of the gall is dependent upon the species of the insect rather than upon the species of the plant. There must be nearly as many different kinds of gall-producing enzymes as there are species of gall wasps. When a single species of gall maker attacks more than one species of oak, the form of the gall is essentially the same on all of the hosts. This point has been so often established (e.g. Cook 1902, Kinsey 1920:365) that it is hardly necessary to accumulate more evidence. Nevertheless, the several cases in Cynips may be cited.

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