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Histological Changes Correlated with Evolutionary Changes of Body Size
Vol. 2, No. 3 (Sep., 1948), pp. 218-230
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2405381
Page Count: 13
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1. The analysis of the alterations of proportion, connected with changes of body size, requires not only the examination of the growth gradients in development but also of the selective influences. A large species is not merely an allometrically changed version of its small ancestor. 2. Evolutionary changes of body size often produce accompanying obligatory histological alterations of structure. An increase of the body size thus produces in several cases new structures, a phenomenon demonstrated for the flight muscles, the middle intestine, and the brain of insects. 3. The differentiation and shape of the corpora pedunculata is not only an index of the evolutionary level of the insect species in question, but it also depends on the body size. The adjacent globuli cells are much more numerous in larger species. 4. In the vertebrate forebrain the histologically recognizable regions of the cortex have different growth ratios. The evolutionary increase in size (Cope's Rule) therefore causes a strong modification of brain structure. 5. In very small species of a group of animals some organs either disappear (e.g., the penis of very small land Prosobranchia, segments of the tarsus in certain insects), or are reduced to the minimum limits. 6. The lower limit of body size within a group of winged insects is in part controlled by the maximum filling of the thorax with flight muscles and of the head capsule with brain. 7. Changes of construction and of function that compensate for structural disadvantages of the minimum size may take place near the lower limit of body size. The development of long fringes on the wings of very small insects of several orders is an example of this phenomenon.
Evolution © 1948 Society for the Study of Evolution