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In this again we see a further extension of accurate measurement: another order of things different in quality began to be treated by a quantitative analysis. Lavoisier's is the greatest name. He gave a clear and logical classification of the chemical elements then known, which served as useful a purpose in that science, as classificatory systems in botany and zoology have done in those cases.
That the mere physiological importance of an organ does not determine its classificatory value, is almost shown by the one fact, that in allied groups, in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different.
No doubt this view of the classificatory importance of organs which are important is generally, but by no means always, true. But their importance for classification, I believe, depends on their greater constancy throughout large groups of species; and this constancy depends on such organs having generally been subjected to less change in the adaptation of the species to their conditions of life.
We shall presently see why embryological characters are of such high classificatory importance. Geographical distribution may sometimes be brought usefully into play in classing large genera, because all the species of the same genus, inhabiting any distinct and isolated region, are in all probability descended from the same parents.
Until failing health forbade work with the microscope, he was continually busy with the rational re-grouping of animal forms. They have done more than all else of their period to rationalize the application of our knowledge of the Vertebrata, and have now left their mark for all time on the history of progress, as embodied in our classificatory systems."
That the mere physiological importance of an organ does not determine its classificatory value, is almost proved by the fact, that in allied groups, in which the same organ, as we have every reason to suppose, has nearly the same physiological value, its classificatory value is widely different.
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