Items related to The barnacles (Cirripedia) Volume 60; contained in...
The barnacles (Cirripedia) Volume 60; contained in the collections of the U. S. National museum - Softcover
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1907 Excerpt: ...distinct as they were formed. Allen (1876, 310) concludes from such evidence that the environmental conditions at the center are the most favorable for the existence of any group, and Adams (1904, 210) that the center of dispersal corresponds to a center of optimum vital conditions. It follows from Adams' standpoint that as a form enters other regions in which there are any changes in the optimum conditions these changes may be considered as unfavorable, whether they are in the way of an increase or decrease, since the zero point of a stimulus corresponds to the vital optimum and is situated between the minimum and maximum, both of which result in death. However this may be, the following facts will stand: (1) That the maximum scutellation and size in the genus Thamnophis occurs at the center of dispersal, and the forms that have been produced in the history of its migration have been formed principally by dwarfing and by a reduction in scutellation; (2) that the variation in the number of scales in the different series is definite and not promiscuous, and is correlated in a remarkable degree with changes in the environment. The development of the different groups has thus been orthogenetic. This is nowhere better shown than in such forms as radix, where, as one passes from the western to the eastern parts of the range, not only the average but also the maximum and minimum numbers of plates in the different series decrease steadily, until the range of butleri is reached; and that this is in some way associated with the environment is shown in the numerous instances in which the transition from the scutellation of one form to that of another takes place rapidly, and along the boundary of different sets of environmental conditions. Even in the latter instance...
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